The 60 ma loop comes from the DC telegraph circuits which could be miles long. In the 1940-50s, when more TTYs were used with voice circuits (telephone modems & radios), the distance to the modem or radio converter (TU) was measured in feet, not miles, and a lower loop current could be used.

All the Kleinschmidt models (designed for the military, starting in the late 1940s) had easily accessible 20/60 ma switches. The switch on the TT-4 was labeled "DC LINE / VOICE FREQ LINE"

In the M28 line, there was typically an "Electrical Service Unit" (ESU) which contained a polar relay that isolated the selector magnet from the external line. Since the SM was on an internal loop, it did not need to easily switched between 20 & 60 ma. I'm not as familiar with the M28 line, but I think there as probably a wiring choice on the terminal strip on the rear wall for the 20/60 ma choice.

Have fun,

Duncan
K2OEQ

On 11/15/2024 12:07 PM, Tom Hunter wrote:

W9DDD,

I have a holding magnet 15 with a 20/60 mil switch. So they did indeed put them on some 15s.

Tom N3CRK

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