Relay Basics
Introduction
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 vacuum tube, transistor and integrated circuit, relays were the perfect on-off switch to build logical circuits using electricity. By far, telephone switching consumed nearly all relay use (some for telegraph) from the 1880's until the late 1970's. Relays made for telephone switching by Siemens & Halske in Germany were used by Konrad Zuse for his Z3 relay-based computer in 1941 [Rojas]. In theory, relays could be the foundational switch to implement a (very very slow) AI inference engine (i.e. ChatGPT, Claude).
In Fig 1, the relay is an unoperated state (A) and an operated state for (B). This relay type was called a flat spring relay based on its design. The flexible arm (armature) moves to make or break contacts depending on if the core is energized or not. Figures 1 and 2 below are from [Keister], the classic book on relay applications as used by the Bell System and others.
Fig 1, The basic flat spring relay-- off and on
The following animation outlines the basics of relay operations.
Fig 2 displays two classic relay form factors from AT&T. The left one is a U-type flat spring relay, while the right one represents the cylinder style 200 series. This second style was popular for Step-by-Step exchange switches worldwide. The 3000 type (no image) was designed by engineers Ray and Biddlecombe, of the British GPO, in cooperation with many providers and contractors. It was adopted as a standard in 1932 [POEEJ-2].
Fig 2, Typical General-Purpose relays
In 1952 the Bell System introduced a new type of relay, the wire-spring type. This type was Western Electric’s major leap forward over the older flat‑spring relay, and it added better reliability, manufacturability, choice, and easier maintenance. Many millions were made annualy. See Figure 3 and the short video below of an AF-22 relay following the dial pulses from a vintage
rotary telephone dial.
Fig 3, AJ-9 type wire-spring relay (author's collection)
How many telephone relays were made from the 1920's until 1975? There is no published worldwide value. But we can estimate from available information. For one data point, from 1910 until 1952 about 320 million were manufactured by Western Electric Company according the Bell System records [Kovalick].
There were about 360 million telephones (including extension and PBX phones), dial by far, worldwide in 1975. These were served by 240 million access lines. This is from AT&T’s annual publication, The World’s Telephones. There is an important metric of about 7 relays per subscriber line, factoring in the many thousands of relays for the route switching per central office [Kovalick]. So, from this we can roughly estimate that about 240*7 = 1.7 billion relays were installed worldwide for telephone switching in 1975. Hence, the total amount manufactured is somewhat greater given the 30-40 year lifetime per relay. The total count manufactured is likely in the range 2+ billion relays from 1920-1975.
Relay circuits
With the basics of relays explained, let's look at combinations of wired relays. Need a multiplexer, demux, bit shifter, comparator, flip flop, adder, ...? Wiring relays together creates any logical circuit desired. Of course, relay-count, power, speed and other factors will limit some functions but this is where having a variety of relay coil types, operate speeds, and contact forms is of value. The Bell System manufactured 1000's of relay types to give engineers a wide choice of design options.
Fig 4 is an example 4 relays (A-D) wired to create a 1-in, 16-out multiplexer. There are 16 combinations of on/off for these 4 relays. This kind of circuit is 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 engaged. Each relay has a different contact configuration. Hence, having a large catalog of relay choices, a relay bible, is essential for designers.
For relay D, the Western Electric (WE) Wire Spring relay model AJ21 has 8 "early break then make" contacts (sometimes called a EBM, form-C or SPDT contact) fits the circuit design needs. Ideally, a different relay would be chosen with one "early break then make" contact for relay A. Most WE telephone relays operated (coil) with 48 volts DC but there were exceptions.
Check out my Youtube page for ~30 videos covering a wide range of switching device demos. The channel is dedicated to the electromechanical devices (1880-1975) used to make telephone calls over different exchange types. Included are many relays, both 1D and 2D linear and rotational talking-path switches, sequence switches, and more.
Fig 4, The basic 4-relay transfer tree [Keister]
References
Britannica Yearbook Of Science And The Future 1973
Keister, Ritchie and Washburn; Members of Bell Laboratories, “The Design of Switching Circuits”, 1951
Kovalick, Al, https://www.calling315.com/relay-logic Fig 11 and https://www.calling315.com/history-of-the-telephone-relay, Endnote B
POEEJ-2: The Introduction of the Standard Telephone Relay, R. W. Palmer, The Post Office Electrical Engineers' Journal, April 1934.
Rojas, Paul, The Design Principles of Konrad Zuse’s Mechanical Computers, 2015