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Relays for Telephone Exchanges 



Relays are discussed here and elsewhere on this site. This coverage is more expanded and provides a backgrounder on relays including some examples of relay circuits as used in working exchanges.




A relay is a remotely controlled switch that can open or close (0/1) contacts when sufficient energy is supplied to its coil of wound wire. The coil becomes an electromagnet when energized.  


Before the transistor and integrated circuit, relays were the perfect on-off switch to build just about any logical circuit from turning on a lamp to building the electromechanical Harvard Mark I computer. So, nearly any logical control problem a telephone engineer desired to solve could be implemented with a combination of wired relays.


The general-purpose telephone-type relay is the workhorse for switching circuits. In Fig 1, the relay is an unoperated state (A) and an operated state for (B). The flexible arm moves to make or break contact depending on if the core is energized or not. Figures 1/2/5 below are from [Keister], the classic book on relay applications as used by the Bell System and others.

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basic electromechanical telephone relay explanation

Fig 1

The basic flat spring relay-- off and on

Fig 2 displays two classic relay form factors from AT&T.  The left one is a U-type flat spring relay, showcased in the Relay Tribute machine, while the right one represents the cylinder style 200 series. This second style was popular for Step-by-Step switches worldwide. 








Fig 2
Typical General-Purpose relays

general purpose telephone relays

Fig 3 illustrates the “3000 style” cylinder relay, a design attributed to the British General Post Office (GPO) and other entities. It is very similar to its cousin, the series 200 style, designed by AT&T. Both were used extensively in Step-by-Step switches. Fig 3A shows examples of early relays used by the GPO. 

The 3000 style alone had 1,300 different varieties. Such diversity allowed designers to select the most suitable relay for specific circuit designs, ensuring optimal performance [Kuhn].  

British PO 3000 style relay image

Fig 3
British Post Office 3000-style relay

By 1970, Western Electric was manufacturing more than 80 million relays per year in over 6,500 varieties. Telephone companies worldwide made meaningful contributions to exchange technology over ~90 year period. 

Here is an interesting exchange metric. The advanced #5 Crossbar exchange required 6.5 relays per subscriber line [Mueller].  Note, this does not mean 6.5 dedicated relays for each subscriber, just an average number counting all the relays in a 10K line exchange. This is ~65K relays for a full 10K line office. Some large exchange buildings contained several 10K line offices with likely > 200K relays in a single building. 

Fig 3A. Examples of early relays and Post Office standard relays [POEEJ]

Click, click,….relay magic

Here is one of the simplest examples of a circuit using a relay. 


                                                    Fig 4
                                     A very simple relay circuit

When the switch closes the lamp goes on. The current through the switch may be much less than that through the lamp. They are independent circuits but share a common battery. You could substitute a motor for the lamp if the contacts could support the higher motor current. This is an important feature of a relay; its contacts can control low or high currents as needed.

Fig 5 is a more complex circuit using 4 relays, A-D. There are 16 combinations of on/off for these 4 relays. This kind of circuit could be used to route the sole input terminal on the left to one of 16 outputs on the right. The ABCD terminal on the bottom right connects to the input terminal only if
 relays A,B,C,D are all on. Each relay has a different contract configuration. Hence, having a large catalog of relay choices, a relay bible, is essential for designers. 

Relay Demo #2 on this site explores the 32 combinations that 5 relays can create, each combo playing a different sound for fun.



                                                        Fig 5
                                      The basic 4-relay transfer tree

At the far end of the scale (~1965), is the #5 Crossbar exchange “Marker”, a relay-based device. It has about 1,500 relays and works to control the routing of calls. It’s an amazing piece of engineering and was considered, in 1938, to be the most complex relay-based circuit ever developed. A Marker is pictured here on this site. 

You may find this interesting.  Fig 6
 lists some relay usage statistics for making a local call on different exchange types. Making a call using a #5 Crossbar office requires 1,278 relays operating to complete a "same office" call.  Notice that the number of “held relays” is 2x for SxS office compared to Crossbar. This is an important metric for efficient exchange operation. Lower is better. 

Making a single long-distance call could involve ~5,000 or more relay operations to forge an end-to-end connection with intermediate toll offices helping route the call. Imagine the combined creative power needed to design and build such a system.

explainer, simple relay logic circuit battery, switch and lamp
explainer, basic 4 relay transfer tree diagram telephpone - 16 combos













Fig 6, from [Mueller]

Counting dial pulses


Next, is an example of combining relays to do a specific operation, counting a caller’s dial pulses. This is a fundamental need for all exchanges.  Let’s look at a circuit to do this. If circuits are not your thing, skip ahead.  


Different dial pulse counting methods were used over the years starting in 1898 after the rotary dial was invented. Below is the dial pulse counting circuit from a Panel office design in 1929. It improved on the method used before it by saving space and resources.


In Fig 7, about sixteen relays are wired to assist in counting the dialed digits. Each relay is shown with its coil and one or more contacts.

Fig 7, from [Raymond]

Dial pulse digit counter using relays


The L relay pulses in sync with the caller’s dial pulses (5 on/off pulses is digit 5). The main counting relays are P1-P6 (and P1'-P6') and these follow a prescribed dance based on each pulse of current from the dial opening and closing.


A “single digit” output value is coded using 5 wires (1/2/4/5/Z). So, a dialed 7 is coded as 2+5. The RA line signals the end of the currently dialed digit. These 6 lines go to a relay-based memory subsystem, not shown, recording all the dialed digits for a given call.


Understanding how these relays respond to each dial pulse from the calling subscriber presents an intriguing puzzle. The puzzle’s solution is Fig 8, a bit of an eye chart, from the original Raymond paper. The code for the dialed digit is displayed in the bottom section. It's likely that Mr. Raymond took pride in designing this pivotal circuit, which was replicated more than 10,000 times in Panel exchanges across North America over the years. It was improved upon for the Crossbar exchange version. 





        Fig 8, from [Raymond Fig 4]

         Dial pulse counting operations

number of telephone relays per call for panel, step and crossbar telephone exchanges
Expaliner, Dial pulse digit counter using relays by raymond for panel exchange
explainer diagram, dial pulse register circuit diagram of operation panel exchange

Relays on display

Below are some closeup images of commonly used exchange relays. These types are also featured on the Tribute to Relays machine. They can be broadly classed into five families based on date of introduction coupled with worthy new features.

These five families are the E, R, U, UB and Wire Spring (WS) and were used by the countless millions in exchanges in North America (NA). In the U family alone, for example, there are about 1,000 individual relay models. Often a new generation replaced the previous one. 

flat spring E type telephone relay 1916
flat spring R type telephone relay 1923

Flat Spring E-1522 (1916)                         Flat Spring R-1842 (1923)      

flat spring U type telephone relay 1938
flat spring UB type telephone relay 1949

Flat Spring U-650 (1938)                         Flat Spring UB-13 (1949)      

wire spring telephone relay AJ-9 western electric

Wire Spring  AJ-9 (1952) 

3 relays e1522 U650 AJ-9 types - hand drawn sketch look

Sketch-look drawings for the E 1522, U 650 and AJ-9  classic relay types

To more fully appreciate relays and associated electro-mechanical rotary switches, check out two 3D videos and an interactive 3D image.



Billions of relays were used worldwide during the electromechanical age of telephone systems. Their contribution to the telephone exchange’s workings is monumental. Here are more closeup shots of a variety of relays standalone and in frames as they were used in actual exchanges. 


Of course, many designers and fabricators were responsible for their overall success. But one designer in particular that left an lasting impact was E.B.Craft of Western Electric. Hats off to Mr. Craft.


To learn more about exchange technology, visit the Exchange Anatomy section.  




Fagen, A History of Engineering and Science in the Bell System: The Early Years (1875-1925), 1978, Bell Telephone Laboratories

Kuhn, W, Critical Relays of the Telephone Systems, Bell Laboratories Record, October 1928

Keister, Ritchie, Washburn, The Design of Switching Circuits, Van Nostrand, 1951


Mueller, "Relays in the Bell System: Facts and Figures," Bell Laboratories Record, June 1957

POEEJ: The Inland Telephone Exchange System, The Post Office Electrical Engineers' Journal, Vol. 49 OCTOBER 1956

Raymond, “All Relay Register Circuit”, Bell Laboratories Record, June 1929

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