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Keith Plunger Line Switch


Switch category: Linear and rotary with multiple movement axis.

Inventor: Alexander Keith (assigned Automatic Electric Company)

Important dates: Line Switch patent filed 1905, US 1,304,324 granted.

Legacy: The Line Switch (or Plunger switch) was a great improvement for the automatic exchange architecture of the day. It saved space and cost with its introduction. It forever changed how exchanges were designed and built.  


Before exploring the structure of the Line Switch, it’s good to review why Keith invented it, its advantages, and how it integrated into an exchange.  

Strowger’s first exchange in La Porte, Indiana, required that each of the 75 subscribers have a dedicated selector switch. Fig 1 shows a 6-subscriber exchange similar in character to what was installed. Binding a switch to every sub is very expensive. Plus, the wiring gets out of hand quickly. For a robust 1,000-line system, there would be ~1 million connections (2 wires each) between switches. 

Keith was the first to introduce a line concentration method that would reduce the costs and complexity of Fig 1. His system did not require a selector switch dedicated to every telephone. Selectors are relatively expensive mechanisms, and they are only used to make a call and not receive one. Plus, only a minority (typically 15% worst case) of subscribers are active at any given time, so a selector-per-subscriber is wasteful.


Strowger early exchange example

Fig 1, Early Strowger 6-line exchange with one Selector per subscriber

Keith invented a new element called the Line Switch (LS) - the concentrator. Adding this type switch to the exchange design reduces overall costs. Let’s explore his method using Fig 2 with 100 stations. The 100 stations on the right and left sides are the same. Each LS has access to only five selectors but not every wire connection is shown. Importantly, there is not a Selector dedicated per subscriber.

step exchange with Keith line switch

    Fig 2, An exchange with a Line Switch

An LS is dedicated to each subscriber. Each LS functions as a pre-selector. What does this mean? Each LS (100 in Fig 2) is assigned the same free selector, from a pool of five in this case, before the next subscriber goes off-hook. When a subscriber does go off-hook, its LS thrusts its plunger into a contact bank to seize the (pre-chosen) selector.  This selector is temporarily assigned to serve the new caller. It provides dial tone and accepts the two dialed digits (in this example) to connect to the called sub.  

Assume sub #1 in Fig 2 goes off-hook and the line circuit (Line relay operates) indicates the caller needs attention. Using circuits not shown, the sub's LS instantly sizes its pre-chosen idle selector. So, sub #1 receives dial tone from selector #2.

Importantly, the Selector can be of any type, but the Strowger type was most common during the early 1900's.


The Keith Line Switch was very successful and used extensively worldwide. Before understanding the LS in more detail, let’s look at a competitive method. This technique attemptes to achieve similar results as the LS but using a different means.   

The Line Finder approach

An alternative line concentration method to the Line Switch was invented by Western Electric (AT&T) engineer Frank McBerty. His method was outlined in US patent 1,117,511 submitted in 1910, granted 1914. He was a major force in the development of the WE Rotary 7A system.


McBerty was certainly aware of Keith’s Line Switch. However, he sought a different way to reduce the number of selectors in an exchange. His patent calls out a “Line Finder” (LF) on its first page and this was a switch.  The patent describes a different use case but the basic idea was extendable. He suggests a rotary switch, but his ideas apply equally when a Strowger or other switch type (panel, crossbar, other) is applied as the Line Finder. Let’s look at a simplified exchange using the Line Finder in Fig 3.

Line Finder section
Line Finder in a telephone exchange

Fig 3, The Line Finder in an exchange blueprint

Notice that the LF did not replace the Line Switch. The layout is distinctly different from Fig 2. No switch is dedicated per-subscriber. This is the biggest difference to Keith’s method. When a sub goes off-hook, the LF is set into motion to “find me”. Here is a typical sequence of operation.

  1. Sub #50 goes off-hook. The Line circuit requests a free LF search for sub #50.  This could be any one of the five LF’s that is free.  

  2. In this case LF #3 does the search and finds sub #50. Each LF is permanently tied to a selector.

  3. Selector #3 provides dial tone and accepts the two dialed digits (in this example) to connect to the called sub.

From the subscriber’s viewpoint, Keith’s or McBerty’s methods present a nearly identical calling experience. So, which method is the better design in terms of cost? See Appendix B. 

As history shows, the Line Finder eventually won the day, especially for medium to large exchanges. Nonetheless, the Keith Line Switch was first on the market, and it set the tone for reducing expensive selectors in an exchange.  The method was popular worldwide during its prime.


Next, let’s look at how it was constructed and functioned.

Inside the Keith Plunger Line Switch

If the switch was a simple 5 or 10-step solo rotary design, this discussion would be much ado about nothing.  The invention is quite the opposite and was one of the most complex exchange switches ever built.  One of Keith’s key ideas was to build a tower of interconnected, ganged, switches all controlled by a common element that he called the “master switch”. Why so complicated? So that the cost per-line was meaningfully less than for a solo line switch.


Fig 4 is a picture of Keith’s composite device with 50 Line Switches. The entire device is called a Line Switch Unit.

Dimensions: 56” (1420 mm) x 21” (545 mm)  x 10” (250 mm), 253 lbs (115 kg).

Keith Line Switch Unit with 50 switches

Fig 4, Bank of 50 Keith Line Switches recovered from a rural exchange in New Zealand, made by the Automatic Electric Company, Chicago, 1920-1925. From London Science Museum.

There are two identified components: an individual Line Switch and the master switch. The 50 individual line switches are apparent.

Fig 5 is a view of a 100-Line Switch system from circa 1912. There are two stacks of 50 LS each. Each stack is a Line Switch Unit. There are two Units and one master switch in the middle of each Unit. Typically, a Unit supported either 25 or 50 individual Line Switches.

100-Keith Line Switches in a frame

                    Fig 5, 100-Line Switches, on hinges to open for repair [McMeen]


Small disclaimer, the explanation to follow takes a few liberties to simplify the basics of operation. Some authors on this same topic use many pages detailing how the LS works, click by click. Not so here. That said, let’s uncover what makes this baby tick.  

Deconstructing the Line Switch Unit

Fig 6 is a high-level view of one quadrant of Fig 5. This supports 25 subscribers with 25 Line Switches and one master switch. There are 10 Selectors shown and these would logically be a resource to all 100 LS in the frame. So, for 100 subscribers, this supports a practical max calling density of 10% of all subs. Again, adding the LS allows the costly Selectors (dial tone and 1st digit at least) to be a shared resource and not dedicated to any subscriber.

Overview of Keith Line Switch in telephone exchange

Fig 6, Overview of Keith’s Line Switch Unit

The Master Switch (MS)

This component could also be called the switch controller. The details of the LS internal workings will be explained later. Using the indicated numerals in the figure, let’s follow a sequence of operations for preparing an LS to connect to a free Selector.  Let’s assume that only Selector #5 is idle (free).

  1. Leads (1) indicate to the MS what Selectors are busy/idle from the pool of ten.

  2. The MS rotates the guide shaft (3) so that all idle LS internal switches (explained below) are pointing to Selector #5, but none seize it. Selector #5 is now pre-selected for use by the next active LS. The MS and all idle LS are in a wait state.  

  3. If sub #K (or any sub) goes off-hook, its LS immediately seizes Selector #5, and #K receives dial tone.

  4. The MS immediately finds another free Selector, if available, and repeats steps 1-3 with the new idle Selector.

The amount of MS control logic for steps 1-3 is minimal, using just a few relays. However, the mechanical workings are non-trivial as will be seen in some images below. 


The Plunger Line Switch internals

Next, let’s explore the LS internals using Fig 7. As already mentioned, this is a high-level view, and some subtle details are missing. The movement of the plunger arm (pivot operation) is simplified. During its time of production, there were several versions of the Keith Line Switch, but the differences are beyond our scope.

As with the Master Switch explanation, let’s follow a sequence of operations using the indicated numerals in the figure. To start, let’s assume the MS begins searching for an idle Selector, and this is selector #5

Fig 7, Simplified individual Line Switch internals

  1. To start the search the Master Switch rotates (arcing motion) the guide shaft (1) relatively quickly, one step at a time searching for a free selector. The shaft extends the length of the Unit and each of the 25 LS is physically coupled to it.

  2. An arc of metal, the fantail, (2) follows the guide shaft’s rotation on all idle Line Switches. So, if all 25 LS are idle every fantail rotates following the movement of the guide shaft.

  3. Attached to the fantail is the plunger arm (3). As the arc turns so does the plunger and its tip (4), an insulated wedge or prong. The MS stops the guide shaft rotation such that the plunger points to free selector #5 on bank (5). The wedge roller (4) does not yet contact the terminals on bank (5). The MS and all LS are now in a waiting state.

  4. If subscriber #1 (or any sub) goes off-hook, the individual LS electromagnet EM (6) energizes, and this moves the pivot arm forcing the attached plunger wedge (4) into the terminal stack (7). At the same time, (3) disconnects from the shaft (1, 2), isolating the LS from the master switch during the call.

  5. The terminal (7) T and R leads are connected to Ts5 and Rs5 respectively by the action of the plunger forcing the contact springs to touch. The T and R leads (tip and ring names) are directly connected to a subscriber’s phone line and the Ts and Rs leads go to the individual selectors. Future versions added a Sleeve (S) lead for control purposes. 

  6. When Sub #1 goes on-hook, the plunger retracts from the terminal stack (7). The contacts are now open. The plunger arm (3) reattaches to the guide shaft (1, 2) ready for another call.

So, that’s it. Now, trying to decipher how Fig 5 works should be a bit easier. One author said Keith’s design “demands admiration”. The choreography is very clever, and the Line Switch Unit greatly advanced the state of the art for exchange design starting circa 1906.


This ends the expository discussion, but you may enjoy seeing the line switch in action with demo videos and a picture gallery of switch images in Appendix A below.


Many thanks to Brian Cameron (at Ferrymead) for providing images and helpful insights.

Thanks to Len Hicken for helpful suggestions on Line Switch operations. 



Hill, R.B., The Early Years of the Strowger System, Bell Telephone Laboratories Record, March 1953

Western Electric, Fundamentals of Telephone Communication Systems
Revised Edition, 1969

McMeen: Telephony - A comprehensive and detailed exposition of the theory and practice of the telephone art, by Samuel G. McMeen and Kempster B. Miller, 1914

Ferrymead Post and Telegraph Historical Society, switch room, Christchurch, New Zealand. 

   -- The Strowger equipment at Ferrymead, retrieved from Whanganui city in 1984, bears resemblance to the General Post Office exchange set up in Wellington, New Zealand, on 20 June 1912. 

                                    Appendix A

                Picture and video gallery of the Keith Plunger Line Switch

Alexander Keith was very creative. He solved the difficult problem of assigning a free selector (provides dial tone) to the next caller using a clockwork-like mechanism.  The videos below show the orchestration of the pieces working mutually together.

This Line Switch is currently in the switch room, Ferrymead Post and Telegraph Historical Society, Christchurch, New Zealand. Thanks to Brian Cameron for the video clips [Ferrymead].

The short video below demonstrates the plunger guide shaft rotating as the master switch searches for idle 1st selectors from a pool of ten. Each time a subscriber goes off hook, the switch searches for a new free 1st selector. This happens three times in the video. 

The image below highlights key components of an Line Switch, including connecting banks, a plunger, and a motion control electromagnet. From [Hill]. The semicircular arc at the top end of the plunger is called the fantail. 

Keith line switch plunger banks and  activating magnet

The image below is a superb “exploded view” showing the main mechanical elements.  At the top is the master switch. This feature drives the plunger guide shaft, and all individual line switches derive their rotary motion from it. The master switch has the logic to find the next free 1st selector and rotates the central shaft accordingly. The ten 1st selectors are shared among all the line switches.  In some cases, these could be 100 line switches. Compare to Fig 6 above. From [Western], edited by author.

The image below is a superb “exploded view” showing the main mechanical elements.  At the top is the master switch. This feature drives the plunger guide shaft, and all individual line switches derive their rotary motion from it. The master switch has the logic to find the next free 1st selector and rotates the central shaft accordingly. The ten 1st selectors are shared among all the line switches.  In some cases, there could be 100 line switches accessing the same ten selectors. Compare to Fig 6 above. From [Western], edited by author.

exploded view of keith line switch showing master switch, plungers and terminal banks

Line Switch Unit diagram

The principal function of the master switch (MS) is to control the rotation of the plunger guide shaft. The power for this movement is furnished by means of a U-shape spring, which, when allowed to act, will rotate the guide shaft in a counter-clockwise direction. The spring and shaft can be seen in the image below [Ferrymead]. The MS starts a new search for an idle 1st selector immediately after a new caller goes off-hook and seizes the currently reserved free selector.   

master switch close up of keith line switch

Portion of the master switch with spring

Below is a close up inside two line switches showing their plungers and terminal banks [Ferrymead]. The plungers are currently off their respective terminal bank.  Each is ready and waiting for an associated subscriber to go off-hook, if ever, then it will “take the plunge.” Plunging connects a sub’s T and R leads to the reserved and idle 1st selector.   

close up of ketih line switch plungers and terminal banks

              Appendix B
   Line Finder Versus Line Switch

And the winner is …

In both cases, as more subscribers are added more LF or LS (plus Selectors) are needed to support the calling traffic without blocking during heavy calling periods.

Incidentally, the "uniselector" (a single, small, rotary switch per sub) was used often in the British Post Office domestic telephone system in place of the Keith Line Switch. The uniselector approach performs Selector hunting as a Line Switch does but each uniselector is a standalone device whereas the Keith Line switch is a composite device as shown in the main text of this section. 

Assume that the cost of a Line Switch per subscriber (or uniselector per sub) is 20% the cost of a Selector and the cost of a Selector and a Line Finder are $100 each. Also, for an exchange with 1,000 subscribers the maximum traffic loading is 10% or 100 callers during peak hours. For sure, there are other factors such as wiring complexity, floor space, power, control circuits and so on. For this example, these second order aspects are not considered.

Here is the lion’s share of costing for three different exchange types:  

Selectors only case: 1,000 Selectors
              Cost_Sel = 1,000 X $100 = $100K

Line Switch case: 100 Selectors, 1,000 Line Switches
              Cost_LS =  $20 X 1,000 + 100 X $100 = $30K

​Line Finder case: 100 Selectors and 100 Line Finders,
              Cost_LF =  $100 X 200 = $20K

For the first case with 1,000 selectors, the per-selector cost would be >> $100 because of needing 2,000 terminals each. This is not practical.  For this basic example the Line Finder approach has lower costs.  This shows the high-cost impact the Line Switch has as subscribers grow.

This does not mean that the Line Switch or uniselector method are always too expensive. For different device costs and/or calling rates, the Line Switch approach may be less costly than Line Finder methods.  That said, the Bell System used Line Finders in their panel and SxS exchange designs. The application of Traffic Engineering was used to make wise choices. 

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