One of my favorite things: a 207-type stepper relay. This relay was located in the panel sender and was electrically in series with the fundamental circuit. As the panel selectors hunted for a terminal, they sent pulses back to the sender. Those pulses caused this relay to open and close once for each pulse. This relay in turn operated the relays in the counting chain that counted the number of pulses that had been sent back. When the counting chain reached ‘0’, the sender was satisfied and the selector up-drive was disengaged, and the selector was held in position.
Because it had such a critical role in making selections, the 207-type relay had to be adjusted to extremely tight tolerances. If it mis-registered a pulse from a selector, the selector would then hunt to the wrong terminal, and the subscriber would get a wrong number, even though they dialed correctly. As such, this relay was adjusted to a non-operate value of 9mA and an operate value of 10mA. That’s just .1 milliamp of difference between operating and not operating!
You can also think of this relay as a simple repeater. It took weak pulses coming back from a distant selector (perhaps miles away) and as it pulsed, it sent strong pulses back to the counting chain, which did not have to be quite as sensitive. This “repeater” functionality is quite clear when comparing the design of the 207 relay to a telegraph repeater from the early 1900s. They’re very similar! It shows how much they relied on existing tried and true technology and applied it to different problems to come up with a solution that worked.
(Above: telegraph repeater relay. Early 1900s. Note similarity to 207 relay!)
