Manual / Guide · 1956
AE Training Bulletin 800: Electrical Principles of Telephony
First booklet of AE's Strowger training series: electrical basics.
Manual / Guide · 1956
First booklet of AE's Strowger training series: electrical basics.
AUTOMATIC ELECTRIC TRAINING SERIES ELECTR PLES OF TELEPHONY TELEPHONE S V S T E M S O R I G I N A T O R S O F T H E D I A L T E L E P H O N E Bulletin This is one of the helpful booklets in the AUTOMATIC ELECTRIC TRAINING SERIES on STROWGER AUTOMATIC TELEPHONE SYSTEMS Electrical Principles of Telephony Mechanical Principles of Telephony Fundamentals of Apparatus and Trunking The Plunger Lineswitch and Associated M a s t e r -Switch Rotary Lineswitch The Connector The Selector Pulse R e p e a t e r s Trunking Power and Supervisory Equipment Party-Line Connectors and Trunk-Hunting Connectors Reverting-Call Methods The T e s t and Verification Switch T r a i n Toll Switch T r a i n Switching Selector -Repeater P r i v a t e Automatic Exchanges with PABX Appendix Community Automatic Exchanges Manual Switchboards Linefinder Switches May we send you o t h e r s pertaining to equipment in your exchange ? CONTENTS Part I .ELEMENTARY ELECTRICITY . . . . . . . . . . . . . . . . . . . .. .. ... ... ... ... ... ... ... ... ... ... ... ... ... ... . .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . .. ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... Electricity and its effects Electrical units Ohm's law Resistors in s e r i e s Resistors in parallel Electric currents A-c circuits Conductors Insulators 1 1 1 1 2 2 3 3 3 P a r t 2 .MAGNETISM . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . .. ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... . . . . . . . . . . . . . . . . . . 10. Magnets 11 Theory of magnetism 12 Electromagnets 1 3 Relays 14 Non-inductive coils 16 Inductors . .. . . 3 4 5 5 7 7 P a r t 3 .SYMBOLS-CIRCUITS-FAULTS ... .. . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . .. .. ... ... ... .. . .. . .. . .. . .. . .. ... ... ... ... ... 16 Symbols 17 Combinations of apparatus 1 8 Methods of circuit control 19 Circuits 20 Describing a circuit Part 4 .. .. 7 8 9 10 .TELEPHONE PARTS . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. . .. ... .. . .. . .. . .. .. . .. . .. . .. . . .. . ... ... ... ... . . . . . . . . . . . . . . . . . . 21 General 22 Permanent-magnet receiver 23 Direct-current receiver 24 Transmitter 25 Polarized ringer 26.Dial . 7 11 11 11 12 12 13 P a r t 5 .TELEPHONES-TELEPHONE SYSTEMS . General . . . . . . . . . . . . . . . . . . Local-battery telephone . . . . . . . . . . . . . . ... Party Local-battery telephone induction coil . . . . . . . . . . . . lines . . . . . . . . . . . . . . . . . . .. Repeating coil . . . . . . . . . . . . . . . . . Common-batterytelephones . . . . . . . . . . . . . . 27 28 29 30 31 32 13 13 13 14 14 15 P a r t 6 .TYPES OF AUTOMATIC TELEPHONE .. General . . . . . . . . . .. .. .. .. . . . . . . . .. .. .. 16 Series automatic telephone 16 . Booster automatic telephone . . . . . . . . . . . . . . 17 . . . . . . . . . . . . . . 17 Anti-sidetone circuit . 33 34 35 36. 37 . Hookswitch . . . . . . . . . . . . . . . . . 18 P a r t 7 .TELEPHONE SWITCHBOARDS .. Manual switchboards . . . . Automatic switchboards . . The lineswitch . . . . ... The linefinder switch The selector . . .. .. .. The connector . . . Lineswitch hundred-line system .. Linefinder hundred-line system Thousand-line system . .. .. Multi-office Ten-thousand-line system system . . . 38 39 40 41 42 43 44 45 46 47 48 a P a r t 8 .BASIC SWITCH-STEPPING CIRCUIT .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Slow relays s o r t dial pulses 50 Release 51 Application . . . . . . . . . . . . . . . . . . 23 24 24 Part 9 .GLOSSARY OF TELEPHONE TERMS Beginson . . . . . . . . . . . . . . . . . . 24 Modern dial telephone (Aiitomatic Electric Company type 80 Monophone) ELECTRICAL PRINCIPLES OF TELEPHONY PART 1 ELEMENTARY ELECTRICITY 1. ELECTRICITY AND ITS EFFECTS "Electricity" is the name given to an invisible agent known only by its effects. The electrical effects with which we a r e most familiar a r e light, heat, power, voice transmission, and many others which we use and depend upon each day. The laws which govern the flow of electricity in closed circuits a r e similar to those which determine the flow of water in a water circuit, Water flowing in a pipe transmits energy; similarly electricity flowing in a conductor transmits energy. The exact nature of electricity is unknown, On the other hand, it is not necessary to know exactly what water is, to use the energy of falling water o r water under pressure to develop power. It is, however, necessary to know how water acts and how it may be used to develop power. Similarly, in the study of practical electricity, the questian of what electricity is, need not be considered; but how it acts and how it can be directed and controlled to do useful work are important. Batteries and generators put electricity into motion or force it to move. An electric generator may be thought of a s a pump which forces electricity to flow within a circuit. 2. ELECTRICAL XJNITS (a) Ampere Electricity in motion is called an electric current. Therefore, a current d electricity in a wire is electricity in motion in that wire. There can be no steady current flow without a complete circuit to and from the source. In formulas, current-flow "intensity" i s represented by the symbol "I". It is measured in "amperes". In telephone work, currents often are small fractions of an ampere and may be expressed in thousandths of an ampere, called "milliamperes". (b) The force which causes electricity to move or to flow is known a s electro-motive force (e.m.f.), pressure, potential difference, or voltage (symbol "E" in formulas). The electrical pnit of pressure is the "volt". If an e.m.f. is applied to a closed or complete conducting circuit if. will force electricity to circulate or flow around in that circuit. The flow of water tbaugh a pipe depends on the pressure forcing the water thru the pipe. Similarly, electrical pressure or e.m.f., measured in volts, causes electricity to flow through conductors_. (c) Ohm Electrical resistance is the opposition offered by an just as friction electric conductor to flow of current of flowing water against the inside of pipes tends to decrease the water flow in a hydraulic circuit. Current in an electric conductor 911 vary in accordance with the resistance offered to its flow. Resistance (symbol "R" in formulas) is measured in "ohms". After numbers, "ohms" often is abbreviated w or more modernly 9. It is evident that the amount of electricity that will flow through a given circuit will be determined not only by the voltage forcing the current, but also by the resistance offered by the conductors. With a specified voltage, the greater the resistance the smaller the current and vice versa. 3. OHM'S LAW A simple but important relation exists between the e.m.f. (volts), the current (amperes), and the impedance (ohms) in any electric circuit. This relation was first discussed mathematically by Dr. Georg S. Ohm in 1827 as follows: The intensity of the current in a direct-current circuit is directly proportional to the voltage and inversely proportional to the sum of the resistances in the circuit. This is known a s "Ohm's Law". The formula for it, and two other formulas derived from it, follow: I = E or, Amperes = Volts Ohms E Volts R = T o r , Ohms = Amperes E = IR or, Volts = Amperes X Ohms - Examples in Ohm's Law Assume that it is desired to ascertain the amount of current that will flow thru a coil having a resistance of 130052 and an impressed e.m.f. of 50 volts. Using the formula I = E E = 50, R = 1300, I = 50 - 0.038 1300 R' ampere or 38 milliamperes. Tf the voltage and current a r e known, to find the resistance use the formula E R = 7. The resistance of a coil thru which 0.06 ampere - -. flows when 50 volts is impressed on it i s found as follows: E = 50, I = 0.06, R = 50 = 8338. 0.06 Knowing current flow and resistance, voltage can be determined. A coil with 120052 resistance has a current of 0.04 ampere flowing thru it. Using the formula E = IR: I = 0,04, R = 1200, E = 0.04 X 1200 = 48 volts. 4, RESISTORS IN SERIES - If a circuit has 2 or more parts in series, the resistance of the whole circuit is the total of the separate resistances. - .. . 50 v o l t s In the sketch above, 500Q + 200052 = 250052 is the totd resistance of the circuit. The current I = E/R = 50/2500 = 0.02 ampere or 20 milliainperes. CURRENT REVERSES DIRECTION + MAXIMUM VALUE ( 1.41 x effective value ) I A EFFECTIVE VALUE \ 5. RESISTORS TN PARALLEL If a circuit has 2 or more loads in parallel, the net resistance of the whole circuit i s the reciprocal of the sum of the reciprocals of the separate resistances. Quite a mathematical mouthful? It looks much simpler, tho, in formula-form: - MAXIMUM VALUE 31 1 This 2 3 TIME 4 5 is a g r a p h i c a l r e p r e s e n t a t i o n of alternating current For example: (b) Alternating Current 0.125 ampere 0 *- T 1 - 50 volts 1 An a l t e r n a t i n g c u r r e n t is one which is constantly r e v e r s i n g i t s direction. A cycle is one complete r e v e r s a l of current flow; that is, a current rising to maximum value in one direction and returning to zero, and then going t o a maximum value in the opposite direction and returning to zero. The number of cycles per second (abbreviated I ) is known a s the frequency. Alternating currents of frequencies from 16- to 67% a r e used to operate telephone ringers. Alternating currents of frequencies up to about 15,000- are audible. Frequencies f r o m 250% to' 3,000- play an important p a r t in telephone transmission. 1 500G 1 - 0.002 + 0.0005 = 0.0025 = +%=E$+z%ij- 1 - R1 400 s o the 2 loads together look t o-the -battery like 400a. Together, the 2 loads draw I = fi. = 400 = 0.125 ampere '" 6 B % from the battery. uA 1 6. ELECTRIC CURRENTS 0 2 1 3 TIME Of the various forms of electric current the four most commonly considered in telephone work are: direct current, alternating current, pulsating current, and undulating current. 7 6 This +- MINIMUM VALUE (Increases and decreases but does not reverse polaiity) 4 5 is a g r a p h i c a l r e p r e s e n t a t i o n of p u l s a t i n g c u r r e n t ( c ) Pulsating Current E-r 5 Z w # 3 ISTEADY ELECTRIC CURRENT FLOW Pulsating current is a current which always flows in one direction, but periodically varies in magnitude so as to flow with a series of pulsations, 2 U 1 O I This 2 3 TIME 4 5 is a g r a p h i c a l r e p r e s e n t a t i o n of d i r e c t c u r r e n t (a) Direct Current A d i r e c t c u r r e n t is one which always flows in one direction and when of constant strength is called a continuous current. The storage battery, the rectifier in various forms, and the direct-current generator are the most commonly lmown sources of direct current used in telephone practice. agnitude changes) #3 (NO -periodic changes) -polarity changes) 4 1 2 This is a g r a p h i c a l 3 TIME of undulating 5 r e p r e s e n t a t ion current 1 (c) Impedance (symbol: "Z") (d) Undulating Current ( Undulating current flows always in one direction but its magnitude changes aperiodically. The current flowing in a telephone transmitter is a typical undulating current. '7. A-C CIRCUITS In rzlternating-current circuits and computations, one must consider not only the resistances in the circuit but also the reactances due to inductances and capacitances. (a) Inductance (symbol: "L") Electricity in motion always produces magnetic flux. When the current in an electric circuit varies, the amount of magnetic flux varies also, increasing when the current increases and decreasing when the current decreases. While changing, these flux changes induce voltage momentarily in any conductor the flux crosses. Thus, when c u r r e n t in a wire is changing, voltages proportional to the changes a r e induced in wires parallel to the current-carrying first wire. This effect i s known a s induction or inductance. p' I. Because a coil consists of many turns of wire close to each other, when varying current flows through a coil, each turn of wire is coupled inductively to and affects every other turn. Thus, the coil has inductance. The inductance of iron-core coils is measured in "henrys" or "henries"; that of air-core coils in "millihenrys" or "millihenries" (thousandths of a henry). (b) Capacitance (symbol: "C") When two parallel conductive plates are close together, if voltage is applied to the plates, they receive and store an electric charge. The pair of plates has been called a condenser or mare modernly a capacitor. The amount of electric charge the plates will hold per volt is expressed a s the capacitance of the capacitor. In calculations, the unit of capacitance is the "farad". A "farad" is quite large, however. Usually capacitors a r e rated in "microfarads" (millionths of a farad). Correct calculations of current flow in alternating-current circuits must take into account not only the resistances but also the reactances (symbol: uX,,) due to inductances and cauacitances in the circuit. Together these make up the t i t a l or net opposition to a - c flow, called "impedance" and (like d-c resistance) measured in "ohms". The calculation of reactance from frequency and inductance o r capacitance, and the calculation of impedance from resistance and reactance a r e beyond the scope of this bulletin. Explanations will be found in any public library in books on physics (shelved under No. 530) or on electricity (shelved under No. 537). Having voltage and impedance, calculate current flow according to Ohm's law: I = E/Z. (d) Effective or "Root-mean-square" Value Alternating currents are defined so as to rrrake applicable to them essentially the same laws that govern HEATING and transfer of power by direct current. An alternating current which produces HEAT in a given resistance at the same average rate as I amperes of direct current is said to have an "effective" or r.m.s. value of I amperes. 8. CONDUCTORS A substance which offers little resistance to the flow of electricity is a good electrical conductor. Most metals are good conductors, copper being one of the best. For this reason copper wire i s .used on relays and magnet windings. German silver wire i s a fair conductor, but offers much greater resistance than copper. It is used in the windings of resistance coils. Most metals i n c r e a s e in r e s i s t a n c e when heated. 9. INSULATORS Extremely poor conductors, such a s rubber, cotton, silk, mica, enamel, and mmy others, are known as insulators, and a r e used to prevent flow of electricity. PART 2 Q i 10. MAGNETS (b) Magnetic Field Any substance which attracts iron and steel i s said to be magnetized or t o possess magnetism, and i s called a ma@et. The following paragraphs define of the most common terms generally used in connection with magnets and explain the various types of magnets. The space immediatelysurrounding the magnet through which the magnetism acts i s called the magnetic field. Magnets a r e classified into two general classes; viz., natural and artificial magnets. (a) Magnetic Poles (c) Natural Magnets The two ends of a magnet where the magnetism is the strongest a r e called magnetic poles. Natural magnets are pieces of iron ore called magnetite or loadstone or lodestone, found in a number of places. The earth itself is a natural magnet with poles near the geographic north and south poles, and with a field covering the entire earth. This enables the compass, which will be explained later, to function to guide people about the earth. Natural magnets a r e not used very &en, therefore it i s not necessary to spend much time studying them. molecules arrange themselves promiscuously in the body; therefore, there is no resulting external magnetism. Thus the steel is said to be demagnetized. When a bar of iron or steel is placed in a magnetic field, those tiny magnets a r r a n g e themselves in a systematic order, with all like poles pointing in the same direction and external magnetism is evident. (d) Artificial Magnets (b) Arrangement of Molecules Artificial or commercial magnets a r e divided into two general groups; viz., permanent magnets and temporary magnets. A permanent magnet is any substance which will maintain indefinitely a magnetic field in the immediate space outside its own volume, without aid from any source. This is true of hardened steel which has been magnetized. Temporary magnets are by far the most extensively used of any of the magnets. Temporary magnets generally take the form of an electromagnet. An electromagnet usually consists of a soft iron core around which t h e r e is wound a number of turns of insulated copper wire. This wire is connected to a source of electric current such as a battery or direct current generator. While the current is flowing, the device becomes a magnet. As soon a s the source of c u r r e n t is discontinued, the magnetism dies away rapidly. T h e r e l a y s u s e d in Strowger Automatic Telephone equipment a r e electromagnets of this type and will be further explained later. (e) Residual Magnetism Residual magnetism is the magnetism retained by the c o r e of a n electromagnet after the circuit has been broken. In order to understand this theory more clearly the case presented by a straight bar of steel should be considered. In its original state the bar is not magnetized, and the molecules a r e arranged in every conceivable direction. As soon as the bar is magnetized, however, the molecules are caused to rearrange themselves in systematic order with all of their north poles pointing in one direction, the south poles in the opposite direction. That pole of the magnet 'towards which the molecular north poles a r e pointed i s called the north pole and the opposite i s called the south pole. If this bar is suspended from a string at its center point i t will turn around until this north points north. If disturbed, it will always come to rest with the same pole pointing north. (c) Compass Such a permanent magnet supported at its-center on a jeweled pivot point is commercially known a s a compass. One end of the magnet, o r needle as it is known in a compass, i s distinctively marked, either by color o r shape, and that end always points north. If a magnet i s dipped into some iron filings, it will be seen that the filings will cling to the ends of the magnet and not to the middle. This shows clearly the presence of the poles which are the points at which the magnetism is the strongest. Magnetism, as previously indicated, may be set up more easily in s o m e m e t a l s than in others. Temporary magnets use soft iron, instead of hardened steel as used in permanent magnets. Soft iron differs from steel in that magnetic lines are set up more easily in iron, and the number of lines set in iron a r e greater than in steel of the same volume. On the other hand, steel may hold its magnetism indefinitely while-iron loses it almost instantly as soon as the current flow ceases. Where more complete absence of residual magnetism is desired, the iron is further softened by annealing. For this reason annealed or soft iron cores are used in the construction of relays f o r Strowger Automatic Telephone apparatus. If the north pole of any magnet is brought near the north pole of a suspended magnet, the latter will turn away. If the south pole of the first magnet is brought near to the north pole of the suspended magnet, the latter will turn toward it. In other words like poles repel, and unlike poles attract each other. This is an important fact and should be remembered. 11. THEORY OF MAGNETISM (e) Geographic Poles It is very difficult indeed to say just what magnetism really is. It is invisible, it has no weight, yet it manifests itself according to very definite and well known laws. Several theories have been advanced to explain magnetism and one of these, the molecular theory, is generally accepted as correct. The north and south poles of a compass and of a magnet have just been defined. Poles of the earth which a r e divided into two classes may be defined as follows: (d) Attraction and Repulsion (a) Molecular Theory Geographic poles a r e those points which represent the position of the north and south extremities of the earth's axis, the imaginary line around which the earth rotates. Thetheory assumes that a bar of steel or iron, composed, like all matter, of small molecules, is made up of minute magnets. If the steel or iron is not magnetized, these Magnetic poles, located in a,position not greatly distant from the geographical poles, a r e the points where the intensity of the e a r t h ' s magnetic lines is greatest. iWWd A north pole of a magnet has been defined as the pole i which seeks the north pole of the earth. Since unlike poles attract each other, it follows that the magnetic pole near the earth's geographic north pole is the magnetic south pole, and the magnetic pole situated near the earth's geographic south pole is the magnetic north pole. In other words: RE Earth's South Pole = Magnetic North Pole. Earth's North Pole = Magnetic South Pole. Hor i z o n t a l T y p e Re lay 13. RELAYS (f) Lines of Force In the cavse of the explanation of the molecular theory, i t was mentioned that when a bar is magnetized the magnetic c i r c u i t is completed both internally and externally. T h i s may be proven. t g% The fact that the lines of force pass through the bar and form a complete loop, and do not start at the north pole and end at the south pole is proven by breaking the bar magnet into s e v e r a l pieces. Each piece becomes a separate magnet with a north pole and a south pole of its own, thus indicating that the lines of force form a complete loop. Lines of force which leave the magnet at the north pole enter the magnet at the south pole, pass through the magnet and return to the north pole, completing the loop. The space outside the magnet occupied by the magnetic lines of force is called the field. If a magnet is placed under a piece of glass on which iron filings have been sprinkled, the filings will arrange themselves along the lines of force, thus indicating the presence and position of the magnetic field. 12. ELECTROMAGNETS Electromaghets a r e in common use in every branch of industry. The iron and steel industries use an electromagnet attached to a crane to hoist large quantities of iron or steel. Every motor has an electromagnet in it, that magnet being known a s the field of the motor. Scores of other uses for electromagnets in equipment varying in size from the door bell to the largest electric locomotive could be mentioned. Strowger Automatic equipment would be impossible without them. ( ; In 1820, H. C. Oe~sted,a European scientist, discovered that there is a magnetic field immediately around a wire through which an electric current is passed; that is, the electric current in the wire produces magnetism around it. This magnetic field lasts as long a s there is a flow of current in the wire. Just why such a thing happens is not known, but it is definitely known that it does happen. The strength of this magnetic field, that is, the amount of magnetism present, depends upon the +number of amperes flowing in the wire, and, when the wire is wound in a coil, the number of turns in the coil. The product of these two factors is known as the ampereturns, a term about which a great deal will be said and which one should be thoroughly familiar with. The relays s o extensively used by Automatic Electric C o m ~ a n vconsist of a coil. o r electromagnet, with a moGti& upon which a movable piece of meLl cbled an a r m a t u r e is attached; also an assembly of springs operated by the movement of the armature. When the relay is not connected to a battery supply, the relay is said to be de-energized, o r a t normal. When in its normal position, the7arma.&re which is hinged, is away f r o m the core; and t h e r e is an a i r gap between the armature and core. When the relay is connected to a batkry supply, a magnetic field is set up in the coil and c o r e which a t t r a c t s the a r m a t u r e to the core. The movement of the armature from its normal position toward the core causes the relay springs to be forced into an operated position. The operation of the relay springs brings about the necessary circuit conditions, which will be explained in detail later. The removal of the battery supply from the relay causes a great reduction of the magnetism which permits the armature to return to normal. Relays could be made to work by having a single turn of wire around the core, but the current consumed would be so great that trouble would result. In order to cut down the number of amperes required, the number of turns of wire is increased. Relays a r e wound with numbers of turns ranging from a few hundred to about ten thousand. The current consumed by such relays varies from about one ampere down t o a few thousandths of a n ampere. (a) Slow Acting Relays Slow-acting relays play an important part in the circuit design of the present day Strowger Automatic Switches. They a r e used to delay momentarily certain circuit operations. Slow acting relays may be divided into two classes; viz., the slow operating relay and the slow releasing relay. This slow acting feature is accomplished by the u s e of a copper collar mounted on the relay, which delays momentarily the operation of the relay armature. (b) Slow Operating Relax The slow operating relay is slow to attract its armature after the circuit has been completed. The copper collar being mounted on the armature end of the relay causes a delay in the attraction of the armature. The action that takes place may be more easily understood when the relay winding is considered as the primary of a trans- - WIRES CROSSING NOT IN ELECTRICAL q e l ~ CONTACT BATTERY CELL GROUND ELECTRICALLY BATTERY WITH GROUND CONNECTION DIRECT CURRENT GENERATOR FIG. 4 FIG. 5 -+A- 0 ALTERNATING CURRENT GENERATOR DIRECT CURRENT MOTOR ALTERNATING CURRENT MOTOR FIG. 6 FIG. 7 FIG. 8 -@- 0 *ymLpnAcT BREAK /IRON SOLDERED LUG TERMINALS LAMP-COMMERCIAL LIGHTING INDUCTIVE COIL SCREW BINDING POST TELEPHONE SWITCHBOARD LAMP FIG. 9 FIG. 10 NON-INDUCTIVE COIL FIG. 3 FIG. 2 FIG. 1 C O R x H A X E COIL TERMINALS CAPACITOR INSULATION FIG. 13 (CONDENSER) @oern" COPPER SLUG AT ARMATURE END FIG. 1 4 FIG. 12 FIG. 11 @ -I- REWY TRANSMITTER COPPER SLUG A.T HEEL . . .- RINGER END FIG. 17 FIG. 16 FIG. 1 5 FIG. 1 8 OFF -NORMAL OR "SHUNT" SPRINGS PULSE SPRINGS HOOKSWITCH RECENER CALLING DEVICE (DIAL) FIG. 19 FIG. 20 Figs, FIG. 21 i t o 23. GENERATOR MAGNETO INDUCTION COIL FIG. 22 FIG. 23 Elementary Symbols f o r m e r and the copper sleeve a s a s h o r t circuited secondary winding consisting of a single turn having a very low resistance. When a voltage i s first applied to the terminals of the winding, the current tends to build up and establish the magnetic field in the relay core. The instant the lines of force cut through the copper collar a voltage is induced in the latter causing a current to flow in i t in the opposite direction from that in the winding. This current in the copper collar s e t s up a field in the s a m e magnetic path, which opposes the field which i s being built up by the current in the relay winding. Gradually the field in the copper collar dies away, and the magnetism due to the winding builds up until it reaches a maximum value and attracts the relay armature. (c) Slow Releasing Relax The slow releasing relay holds its armature, momentarily, after the circuit to the relay has been opened. In this case, the copper collar i s mounted on the heel end of the relay. When the circuit to the coil is broken the magnetic field in collapsing s e t s up a current in the copper collar. This current is in such a direction as to try to maintain the existing magnetism. But since this current itself depends upon the decreasing magnetism, both the field in the copper collar and the relay gradually die away. This action delays the release of the armature. 14, NON-INDUCTIVE COILS The direction of the magnetic field about a wire depends on the direction of the current flow in that wire. There- fore, the magnetic fields of two c u r r e n t s flowing in opposite directions oppose each other and will neutralize each other if they a r e of equal strength and if the wires a r e near enough together. Use is made of this fact by winding two wires into a single coil so that the current in one wire flows in the opposite direction to the current flowing in the other wire, around the coil. Such a coil has no perceptible magnetic field and i s said to be noninductively wound. 15. INDUCTORS ("impedance coils", "retardation coils", "choke coils") Alternating current finds it more difficult to pass through an inductively wound coil than it does through a noninductive coil of equal resistance. A different effect is produced when an inductively wound coil i s placed in an alternating current circuit than when placed in a direct current circuit. In an A.C. circuit a counter e.m.f. is set up which acts a s a "choke7' to the impressed electromotive force. This effect is known as inductive reactance and the combination of reactance and resistance is known a s impedance. Impedance coils are often used in telephone practice to prevent the flow of high frequency currents. The action which takes place in an impedance coil may be described a s follows: the magnetic field around the coil, rapidly changing in direction at each cycle, causes a counter electromotive force (c.e.m.f.) to be set up, which opposes the impressed e.m.f. The effect of this c.e.m.f. coupled with the resistance of the coil jointly limits the flow of current. PART 3 SYMBOLS--CIRCUITS--FAULTS 16. SYMBOLS * @ Electrical circuits a r e explained by a form of writing which employs symbols arranged in such a way a s to tell the reader the electrical conditions and relations which exist. These records a r e valuable because they enable people to duplicate past work, to study the action of electrical apparatus, and to make easier the detection of troubles. The symbols were derived from rude pictures of the apparatus, which have in course of time been simplified t o their p r e s e n t form. Many times the present symbol does not look at all like the object which it represents, but it usually suggests the original form of the apparatus. The apparatus has developed into modern form and the symbol has developed toward simplicity. (Figs. 1 to 23.) Frg. 24. Ser i c s Combtnat ton (a) Series Connection 17. COMBINATIONS OF APPARATUS Some of the most common combinations of apparatus a r e explained in the following paragraphs. A series connection is a method of connecting the parts of an electric system so that the same current passes through each device in the circuit, one after the other in direct succession. (Fig.24.) 18. METHODS OF CIRCUIT CONTROL There are many methods of circuit control. A few of the most common are described briefly below. Fig. 25 >f P a r a 1 le 1 C o m b i n a t i o n (b) Parallel Connection A parallel connection is a method of connecting the leads of an electric system in which all the positive terminals are joined to one conductor, and all the negative terminals to the other. When the pieces a r e thus connected the current is divided between them, and they are said to be in parallel with one another. Multiple or shunt are other names for this combination. (Fig. 25.) Fig. 26. Fig. 28. Break Control (a) Break Control As an illustration of break control assume that three lanips a r e arranged to be lighted by one generator and each lamp is controlled by a switch. Any one lamp could then be cut off by breaking or opening its circuit a t the switch without interfering with other lamps of the circuit. (Fig. 28.) Parallel Groups Connected In Series Fig. 29. Short-circuit C o n t r o 1. (c) Series Parallel Connection (b) Short-circuit Control When devices such a s lamps, in an electric circuit, a r e arranged in groups and connected in parallel in each group, these groups in turn being connected in series, the arrangement is known as a series parallel connection. (Fig. 26.) A short-circuit control arrangement might, for example, consist of an inductive coil in series with a non-inductive coil. The latter could be removed from the circuit by short-circuiting it by means of the switch. This would not cut off the inductive coil. (Fig. 29.) Fig. 27. Series Groups Connected In Paralzel F-ig. 30. Substitution Switching B y Breaking (d) Parallel Series Connection (c) Substitution Switching by Breaking The parallel series connection is a method of connecting groups of devices such as cells, in an electric system so that these cells a r e in s e r i e s in each group and the groups in turn a r e connected in parallel. (Fig. 27.) Switching by breaking may be illustrated by having two inductive coils controlled by a switch whose main spring may connect the battery circuit through either coil by breaking the circuit to the first coil. (Fig. 30.) $ -3 If the r e s i s t a n c e of this path i s high enough not to interfere to any extent with the operation of the coil, the path is called a "leak''. - I - + If the resistance is low enough to exclude practically all of the current from the coil, the fault is called a "short circuit". I Fig. 31. Switching b y Short-circuiting. b (d) Switching by Short-circuiting 1 If two coils a r e wired in s e r i e s (Fig. 31), either coil may be short-circuited by the switch whose moving spring may touch either contact. * I Fig. 33. Grounded Circuit 19. CIRCUITS (d) Grounded Circuits In order for electric current to flow there must be a complete path, that is, a closed circuit; and this closed circuit must include the battery or whatever source of current is used. A break in a wire, or other conductor, will prevent the flow of electricity past that part of the circuit i n which the break occurs. When tracing a circuit, start at one terminal of the battery and follow the circuit through wires, connections, and apparatus until you reach the opposite terminal of the same battery. (a) Wiring and Schematic Diagrams Electric circuits a r e shown in two general classes, wiring diagrams and schematic diagrams. A wiring diagram shows the apparatus approximately i n its relative location. I t i s used mostly for reference when wiring the apparatus and tracing connections. A schematic diagram is electrically the same a s above, but the parts a r e rearranged in such a way a s to show more clearly the paths of current flow, the relation of the apparatus, and the principles underlying the design of the circuit. A schematic diagram may contain all the circuits of a layout, o r i t may contain only one circuit path, rearranged to show itself m o r e clearly. (b) Circuits and Circuit Faults In studying the descriptions of the following circuits and circuit faults, the illustrations should be referred to. Fig. 33 shows a complete circuit with an accidental ground indicated by the dotted line. This ground would not affect the circuit and will not interfere with the current flow o r the operation of the coil, because no other part of the circuit is grounded. Fig. Leak And Short Circuit (c) Leak and Short Circuit If there is an additional path in the circuit, Fig. 32, through which electricity can flow, as shown by the dotted line, s o m e of the current will go through it. Grounded Circuit Fig. 34 shows a complete circuit with the positive side of the battery grounded. Should an accidental ground occur on the positive side of this cii-cuit, a s shown by the dotted line, it will not affect the current flow to the coil, or its operation. Fig. F i g . 32. 34. 35. Grounded Circuit Fig, 35 shows a complete circuit with the positive side of the battery grounded. Should an accidental ground occur on the negative side of this circuit, a s shown by the dotted line, the coil will be shunted out by having a ground on bbth sides of its winding. The battery will also be short circuited as the ground i s on the negative side of the line. F i g . 39: F i g . 36: Fig. 36 shows an accidental ground on one side of the inductive coil. This would cause the coil to be operated at all times, regardless of the position of the switch, since there i s a complete circuit from the accidental ground through the coil to negative battery. F i g . 37: Alternating Current Circuit Grounded C i r c u i t (g) Alternating Current Circuit Since a condenser does not prevent the flow of current in a n A.C. circuit, we can t r a c e a complete circuit, Fig. 39, from one side of the generator, through the non-inductive coil, through the condenser, to the other side of the generator. Cross Circuit ,AY (e) Crossed Circuit The crossed circuit, Fig. 37, a s indicated by the dotted line, would cause both coils to operate when either switch makes contact because a circuit is completed to both coils by the closing of either switch. KEY Fig. 40: C o m p l e t e R e l a y And L a r p C i r c u i t 20. DESCRIBING A CIRCUIT F i g . 38: Incomplete Direct Current C i r c u i t (f) Incomplete D.C. Circuit A condenser in s e r i e s with a coil in a direct current circuit, a s shown in Fig. 38, will prevent the flow of current in the coil. The condenser in a D.C. circuit acts the same a s an open conductor. A.C. but not D.C. will flow through a condenser. In the description of a circuit, c a r e should be used to s t a t e exactly what is meant. Always s t a t e which polarity of battery is referred to, and when speaking of any point in a circuit, define that point s o that there will be no doubt as to what is meant. For example, Fig. 40 may be described as follows: By closing the key there i s a complete circuit from negative battery through the relay, and through the closed key to ground (positive battery). This will cause the relay to operate, which completes the circuit from negative battery through the lamp, and through the closed relay contacts to ground, causing the lamp to light. T h e r e l a y will remain operated and the lamp will remain lighted, as long as the key is closed. BART 4 TELEPHONE PARTS 21. GENERAL In o r d e r t o bring out the principles underlying the transmission of sound waves in telephony, the following definitions a r e given: complete vibration or cycle during the period that one complete cycle of qternating current flows through the magnet coils. A telephone transmitter is that portion of a telephone by means of which sound waves from the voice are converted into e l e c t r i c a l energy which can be transmitted to another point on the line for reception by means of a receiver. A telephone receiver is that portion of a telephone by means of which the electrical energy from the transmitter is converted back into audible sound waves. f OIL CUP IAPHRAGM PERMANENT Fig. 42: Direct Current Receiver 23. DIRECT CURRENT RECEIVER POLE PIECES DIAPHRAGM Fig. 41: Bi - Polar Receiver 22. PERMANENT MAGNET RECEIVER The receiver, as shown in Fig. 41, includes a permanent magnet, magnet coils, and a diaphragm. T h e permanent magnet attracts the diachragm and places a certain tension upon it. One-half cycle of alternating current flowing through the magnet coils adds to the pull on the diaphragm that i s exerted by the permanent magnets, and the other half cycle subtracts from their pull; therefore, the diaphragm makes one Fig. 42 shows a schematic drawing of a direct current receiver. In common battery systems where the direct current is fed from the Central office to