I have collected a few telegraph sounders and my question is how to tell
what DC voltage to use based on the ohms. The 5 ohm and 30 ohm work fine
on a 3 volt battery but I have just picked up a 140 ohm sounder and have
no idea on how to find out the voltage needed to make it work as 3 volts
DC is not enough. Is there a way to find out this type of information as
I am only using a short piece of copper wire and dry cells attached to a
key and sounder. Any help apprreciated.

Tom

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Well we could go at this proportionately. 3 volts and a 4 ohm sounder
gives you (V*V)/R 2-1/4 watts. So for a 140 ohm sounder the square
of the voltage is about the square root of (140 * 2.25) or about 18
volts.

You may find the higher resistance sounder is sluggish in operation,
because of its higher inductance. In telegraph practice the low-resistance
sounders were used with a cell or two of battery in a local circuit operated
by a line relay. The higher resistance sounders were used in the line
circuit where there is a lot of resistance in the line and a considerably
higher working voltage.
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jhaynes at alumni dot uark dot edu