J.H. Bunnell equipment is the finest made.

On the top of the wood base of the instrument somewhere near the

electrical connection terminals you should find a number

stamped there. Look for a number or numbers like "4" "5"

"20" or "150"...or something similar.

This stamped number usually refers to the DC resistance of the electromagnet coil windings in Ohms on that instrument.

"local" instruments such as local sounders and pony relays were usually wound with much heavier wire than "mainline" instruments. The number "4" for instance would refer to a winding total resistance of Four Ohms.

These instruments were designed to use a low voltage like one to two Volts, and require 200 to 400 Milliamperes current in the circuit for the instrument to operate correctly.

A number like "20" or "30" indicates a winding resistance of 20 or 30 Ohms, and the instrument will require 90 to 110 Milliamperes of current to operate.

If the number is "150" or "140" or some similar number, the instrument will require around 40 to 60 Milliamperes of current to operate.

The amount of voltage required to operate a telegraph circuit is determined by the current required by the particular instruments in the circuit and the total winding resistance of all the instruments in series connected in the circuit. The interconnecting wiring in simple "local" circuits can be assumed to have negligible resistance.

Use Ohms Law, which states that (for a series DC circuit) the circuit current in Amperes multiplied by the total resistance in Ohms of the circuit is equal to the Voltage in Volts required.

For telegraph purposes, we use current values measured in "Milliamperes" which is 1/1000 Amperes....(Example 50 Milliamperes is equal to 0.050 Amperes).

Lets say you have a four or Five Ohm telegraph instrument you wish to power up and use. It needs 200 to 400 Milliamperes of current, or 0.200 to 0.400 Ampere. Lets shoot for 300 Milliamperes.

Volts = Current x Resistance (Ohms Law)

therefore

Volts required = 0.300 x 4 = 1.2 Volts

A single 1.5 volt drycell will power a single 4 or 5 Ohm sounder quite nicely...the current will be about 400 Milliamperes, but the heavier wire in the instrument windings won't be damaged by the slight excess of current.

A voltage source less than one volt is hard to come by.

You might want to parallel several 1.5 Volt battery cells to allow the batteries to last longer, but the supply voltage should be not more than 1.5 volts maximum.

In the olden days, "local" sounders in telegraph offices were typically powered by two jars of Gravity or "bluestone" cells in series, about two volts.

If you have a 150 ohm instrument, you want the current to be about

45 Milliamperes plus or minus 5 Ma or so...

Voltage required = 0.045 Amperes x 150 Ohms = 6.75 Volts

For a simple circuit with one 150 Ohm instrument in it, a 6 Volt supply will work it OK. You could use four 1.5 volt drycells in series for a total of 6 volts, or use a single 6 volt lantern battery.

You may have an instrument marked "400 Ohms". These usually have four connecting terminals on the wood base. Typically you will find the center two strapped together, and the circuit connections will be on the outside two posts.

A 400 ohm sounder needs about 35 Ma to operate, so the voltage required would be

Voltage = 0.035 amperes x 400 Ohms or 14 Volts. A 12 volt supply would

give you a current of 30 Ma and would work OK for a single instrument in the circuit. Put two 6 volt lantern batteries in series for 12 volts.

Maybe this will guide you in what you need.

Let us know if you have other questions.

Edited 2 time(s). Last edit at 12/28/2009 12:01AM by Etrump.