The Two-Motion Switch — Telephone Switching History

Figure 1 — An Automatic Electric selector from the demonstration band at the Telephone Museum, Waltham. The dust cover is clear, so the whole mechanism shows: the relays on the switch base at the top, the ver… — Figure 1 — An Automatic Electric selector from the demonstration band at the Telephone Museum, Waltham. The dust cover is clear, so the whole mechanism shows: the relays on the switch base at the top, the vertical shaft with its ratchets in the middle, and the semicircular bank at the bottom, where the wipers do their work. — Daderot, via Wikimedia Commons (CC0)

3.1 One Shaft, Two Motions, a Hundred Lines

Every two-motion switch, whether it is working as a line finder, a selector or a connector, is the same machine. A vertical shaft carries a set of wipers. Below or beside the shaft sits a bank: ten semicircular levels, stacked one above another, each with ten contacts around its arc. Lifting the shaft chooses a level, and turning it chooses a contact on that level, so one shaft reaches 10 × 10 = 100 positions. The switch’s job in the network depends only on what is wired to those positions and on the relays mounted on its base. A connector’s bank positions are subscribers’ lines. A selector’s positions are trunks to the next switch. A line finder’s positions are the lines it may have to answer.

This volume takes the mechanism apart. The descriptions follow the US Air Force correspondence course Telephone Switching Equipment Repairman (Electromechanical) (CDC 36251, republished by ERIC as ED194781), whose second volume walks through the Automatic Electric-pattern switch part by part, and Doug Kerr’s step-by-step papers, written from Bell System practice (USAF CDC 36251, Vol. 2; Kerr, The connector switch). Automatic Electric and Western Electric two-motion switch parts were generally interchangeable, and Western Electric kept AE’s switch-wiring colour code (Kerr, Overview). With minor differences, the description fits both makers’ switches.

Figure 2 — The two motions. Side view (left): the vertical magnet lifts the shaft one level per pulse; the rotary magnet turns it; the release magnet trips the detents; the off-normal springs change over on t… — Figure 2 — The two motions. Side view (left): the vertical magnet lifts the shaft one level per pulse; the rotary magnet turns it; the release magnet trips the detents; the off-normal springs change over on the first step. Top view (right): one level of the bank, with the wipers swung from the normal position to contact 4. Stepping past contact 10 reaches the 11th position, where a selector signals that every trunk is busy. — Original diagram drawn for this dive

3.2 The Parts

Table 1 — The Parts

Part	What it does
Shaft	Carries the wipers. It has two ratchets: a vertical ratchet of horizontal teeth for the lifting motion and a rotary ratchet of vertical teeth for the turning motion.
Vertical magnet	Two coils in series. Its armature carries the vertical pawl, which lifts the shaft one tooth (one level) each time the magnet operates.
Rotary magnet	Two coils in series. Its pawl turns the rotary ratchet one tooth (one contact) per operation. A stop pin keeps the rotary pawl clear of the shaft during vertical stepping, and a flat spring restores the armature against an adjusting screw.
Double dog (double detent)	A pivoted piece with a vertical detent that catches under a vertical ratchet tooth and a rotary detent that holds the turned position. Its spring presses it onto the ratchets while the switch steps. When the switch is idle, the release link holds it clear. Its whole purpose is to stop the shaft falling back while it is being stepped.
Stationary detent	Takes the weight of the shaft during rotary stepping. A groove in the vertical ratchet lets it pass while the shaft moves up and down; the first rotary step moves a tooth over it.
Release magnet	A single coil. Its armature strikes the double dog, pivoting both detents out of the ratchets, and the release link latches the dog clear. The helical shaft spring, wound up by the rotary motion, turns the shaft back to its rotary home; the stationary detent then drops into the ratchet groove, and gravity drops the shaft to normal.
Vertical off-normal (VON) springs	Operated mechanically on the first vertical step and held operated until the shaft is home again. They prepare the release-magnet circuit and switch the relay logic between “idle” and “in use”.
Rotary off-normal (RON) contacts	Operated whenever the shaft is off its rotary home. Used by level-hunting connectors.
Cam springs / 11th-step springs	A cam, or “fin”, on the shaft operates them when the wipers step off the end of a level to rotary position 11. In a selector they switch the caller from dial tone to all-trunks-busy tone.
Rotary interrupter springs	On selectors and line finders only. They are opened by the rotary magnet’s own stroke and let the switch step itself repeatedly to hunt. A connector has none, “since its vertical and rotary movements are under the control of the dial”.
Wipers	Spring pairs with flattened tips, wired to the switch through flexible tinsel cords built to stand millions of flexing cycles. At normal they rest one step below level 1 and one step to the left of contact 1.

The table follows the USAF course, Volumes 2 and 3, and Kerr’s papers. It also notes optional normal-post springs, operated by a cam riding up with the shaft, which let a first selector bar chosen levels to restricted lines, and a release alarm relay in series with the release magnet that raises an alarm if a switch fails to return to normal.

3.2.1 Release Is Not Stepping Home

A two-motion switch does not step backwards to normal. It rotates home, then drops. The release magnet knocks out the detents; the wound helical spring spins the shaft back to rotary home; the shaft falls under its own weight to vertical home. The off-normal springs restore as it lands. British 2000-type selectors are reported to have released differently, continuing forward off the end of the level before dropping (a detail not confirmed for this dive). Automatic Electric switches use the release magnet.

Figure 3 — A Strowger two-motion selector on display at the Musée régional des télécommunications en Flandres, Marcq-en-Barœul. The vertical shaft with its long rotary ratchet runs down the centre; the magnet… — Figure 3 — A Strowger two-motion selector on display at the Musée régional des télécommunications en Flandres, Marcq-en-Barœul. The vertical shaft with its long rotary ratchet runs down the centre; the magnets are at the top and the bank at the bottom. — Pierre André, via Wikimedia Commons (CC BY-SA 4.0)

3.3 The Bank

Each level is a semicircle of ten contacts, and ten levels make a hundred positions. The contacts are brass strips separated by insulation; their tails, out the back, are the soldering terminals.

A switch handles three conductors per position, so a bank position is really a set of contacts, and the switch carries one wiper for each:

Tip and ring, the two wires of the talking pair. Automatic Electric’s literature marks the line bank pairs “positive (+)” and “negative (−)”: the lower contact of each pair is + (tip, the grounded side) and the upper is − (ring, the battery side).
Sleeve, which AE calls the control or C lead, sometimes “private”. It carries no speech. It carries ground, and ground on a sleeve means busy. Some switches carry extra control conductors (EC, or SPL C) for features such as restricted service.

Kerr describes a 100-point connector bank as two stacked banks: the lower carries tip and ring as two thin contacts on either side of a phenolic sheet; the upper carries the sleeve as a single, thicker contact. The tip/ring wiper’s two leaves are insulated from each other; the sleeve wiper’s are tied together.

Two-hundred-point switches exist too. A 200-point line finder has three banks: two carrying the tip and ring of two unrelated lines, and a third carrying both lines’ sleeves. A relay in the finder chooses the upper or lower line. Line finders also have a vertical bank (or vertical commutator) of eleven contacts read by a single wiper mounted at right angles to the shaft. It is how the finder knows which level the calling line is on. Contact 11, the home position, is dead (Kerr, The line finder switch).

3.3.1 Numbering the Positions

The dialled digit and the bank position are the same thing. Dial 1 and the shaft rises one level; dial 0, which sends ten pulses, and it rises to the tenth level. On a connector, the positions on level 1 are therefore numbered 11, 12 … 19, 10, and those on level 0 are 01 … 09, 00 (Kerr, The line finder switch). A two-digit connector reaches 100 numbers with no arithmetic at all.

Figure 4 — A connector bank before wiring, from Smith and Campbell's Automatic Telephony (1921): the semicircular arcs of contacts the wipers sweep, stacked level on level, with the soldering tails out the back. — A. B. Smith and W. L. Campbell, Automatic Telephony (1921), via Internet Archive

3.4 The Multiple

A single switch reaching a hundred lines would be of little use if only one call could use those hundred lines at a time. So the banks of every switch in a group are wired in parallel: contact 23 on one connector is joined to contact 23 on every other connector in the group, and to line 23. Smith and Campbell describe it from the bench: the twisted pair of wires soldered to pair 01 of one bank “loops over” and is soldered to the corresponding pair of each of the other banks (Automatic Telephony, 1921, p. 56). This is the bank multiple, the automatic successor of Firman’s multiple switchboard (Volume 1).

The multiple is also what makes the busy test work. Ground placed on a sleeve contact by any switch in the group appears on that contact in every bank, so every other switch that tests it sees the line or trunk as busy. The USAF course puts it in a phrase: a grounded sleeve is “busy to all searching selectors.”

Smith and Campbell’s wiring vocabulary is still the clearest:

bank wiring: the multiple among adjacent switches;
selector bank multiple: the wires joining different selector groups;
trunks: wires from a switch to the next kind of switch ahead;
normals: wires from connector banks to subscriber lines.

Figure 5 — The Waltham demonstration band with its telephone, mounted against a red board: line finder, selectors and connector side by side, and below them the cabled bank multiple that joins their banks to … — Figure 5 — The Waltham demonstration band with its telephone, mounted against a red board: line finder, selectors and connector side by side, and below them the cabled bank multiple that joins their banks to one another and to the lines. — Daderot, via Wikimedia Commons (CC0)

3.5 The Eleventh Step

A selector sent to a level hunts along it for an idle trunk. If every trunk on the level is busy, it steps off the end of the bank to the 11th rotary position. There the cam springs operate. They transfer the A relay from dial tone to all-trunks-busy tone, which Kerr calls “fast busy”, “two beeps per second rather than one” (Kerr, The selector switch), and stop the cut-through relay from operating. If a level is equipped with fewer than ten trunks, “the vacant contacts are grounded at the control bank to permit the wipers to step past them” (USAF course, Vol. 3). The vacant positions simply look busy.

A line finder that steps to the 11th position without finding its line releases and lets another finder try. A finder stuck at the 11th step with its release magnet not operating is one of the USAF course’s adjustment faults (Volume 8).

3.6 How Fast

A two-motion switch on dial pulses steps at the dial’s speed, about ten steps a second (Kerr, Overview). When it hunts on its own it is faster, since it is limited only by how quickly the rotary magnet and its interrupter can chase each other, but no reliable published figure for Automatic Electric’s two-motion selectors was found. For comparison, AE’s own bulletin on its single-motion rotary switches gives about 50 steps a second self-interrupted for its 25-point Type 13 and 11-point Type 26, and about 65 for its Type 44 and 45 (AE Technical Bulletin 961-473, 1961).

3.7 Single-Motion Switches

Not every switch in a step-by-step office is two-motion. Several jobs need only one row of contacts:

The rotary line switch (and the British uniselector) is “a non-numerical switch, attached to the subscriber line, which can connect the line to a trunk leading to an idle connector” (Smith and Campbell, p. 7). A 25-trunk rotary line switch group could serve 200 or more lines.
The Keith plunger line switch did the same job with a plunger thrust into a bank, steered by a shared master switch that kept every idle plunger aimed at an idle trunk (Volume 2).
Automatic Electric’s minor switch is a small direct-drive rotary with one to three levels, used for jobs such as registering a digit or choosing a ringing frequency. Its name comes from “its original use as an auxiliary or minor switch in telephone systems.”
AE’s numbered rotary family covers 10-, 11-, 25-, 26- and 50-point switches (Types 40, 26/44, 13, 45 and 14). Despite the name, AE’s “Type 45” is a 26-point rotary switch, not a two-motion switch (AE TB 961-473). The distributor in a line-finder group (Volume 4) is a 25-point rotary switch and may be one of these, though no source names the type.

Rotary switches come with bridging wipers, which make the next contact before leaving the last, for circuits that must never open, or non-bridging wipers, which break first so adjacent circuits are never joined (AE TB 961-473).

Figure 6 — An Automatic Electric rotary stepping switch of 1958, Telephone Museum, Waltham. Commons titles it a Strowger switch, but it is a single-motion rotary: one arc of contacts swept by wipers driven ro… — Figure 6 — An Automatic Electric rotary stepping switch of 1958, Telephone Museum, Waltham. Commons titles it a Strowger switch, but it is a single-motion rotary: one arc of contacts swept by wipers driven round by a stepping magnet. Switches of this family served as line switches, distributors and minor switches. — Daderot, via Wikimedia Commons (CC0)

Figure 7 — An Automatic Electric rotary line switch with its bank, from Smith and Campbell (1921). One per subscriber: when the subscriber lifts the receiver, it steps round the bank to an idle trunk. — A. B. Smith and W. L. Campbell, Automatic Telephony (1921), via Internet Archive

3.8 Relays on the Switch Base

The two magnets do the moving; the relays on the switch base do the deciding. Their contact springs carry every decision a selector or connector makes: whether it is seized, whether a digit has ended, whether a trunk is busy, whether ringing has been answered. Two kinds matter especially:

Slow-release relays. A copper slug on the heel end of the core keeps the relay operated for a fraction of a second after its current stops. That is long enough to ride through the dial’s open periods. Smith and Campbell (1921) say why it matters: “the introduction of these slow-acting relays makes possible the operation of the switches by simply making and breaking the calling party’s loop.” The selector’s B (hold) and C (series) relays are slow-release, and Volume 5 shows them at work.
Two-step relays. Automatic Electric’s line-and-cutoff relay closes its make contacts on a weak current (the line relay function) and opens its break contacts only on a stronger one (the cut-off function). The connector’s H relay works the same way.

The coil values quoted in the USAF course’s example circuits give a sense of scale. The selector’s A relay has two 200 Ω windings. The cut-through relay F has 1800 Ω, the auto-rotary relay E 210 Ω, and a connector rotary magnet 57 Ω. At 48 V, that 57 Ω magnet draws about 0.84 A while it is operated. Those values come from the course’s examples, not from any particular AE drawing. Before using any of them on a real switch, check its own circuit drawing.

Figure 8 — The British "Strowger type relay" in cut-away, Fig. 139 of J. Atkinson's Telephony, Vol. 1 (1947): armature, spring set, residual screw and coil. Automatic Electric's own relays, the Class B among … — Figure 8 — The British "Strowger type relay" in cut-away, Fig. 139 of J. Atkinson's Telephony, Vol. 1 (1947): armature, spring set, residual screw and coil. Automatic Electric's own relays, the Class B among them, differ in detail; no comparable published drawing of the AE Class B was found. — J. Atkinson, Telephony, Vol. 1 (Pitman, 1947), via Internet Archive

3.9 In the Reference Library

Sources

USAF Extension Course Institute, CDC 36251, Telephone Switching Equipment Repairman (Electromechanical), Vols. 1–5 (Vol. 3 current to December 1978), ERIC ED194781. https://archive.org/details/ERIC_ED194781
D. A. Kerr, The step-by-step telephone switching system: Overview, The line finder switch, The selector switch, The connector switch. http://dougkerr.net/Pumpkin/
A. B. Smith and W. L. Campbell, Automatic Telephony, 2nd ed., McGraw-Hill, 1921. https://archive.org/details/automatictelepho00smit
Automatic Electric, Technical Bulletin 961-473, Rotary Stepping Switches (1961). https://archive.org/details/how-to-use-rotary-stepping-switches-automatic-electric-1964