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A Military Puzzle

Purchasing a rusty metal box with an unusual hand wound generator and an obscure bakelite handset inside almost turned out to be more than the proverbial jig-saw puzzle.

A damaged metal label gave the first clue with the information that it was made by T.M.C. (Telephone Manufacturing Company) in 1959.

The hand wound generator was soon pulled apart to reveal a mechanism quite unlike a normal phone generator. With a train of gears to give fairley high speed to the circular armature and revealing a circular magnet - it was time to test out. A meter read about 18 volts AC but this output would would not energise any type of ordinary phone bells.

Dismantling the handset with its heavy duty rubber cords revealed a capsule type transmitter and a capsule also in the earpiece. After replacing a broken wire to the earpiece, I joined the two handset wires to the generator and witnessed a surprising result - a very loud siren sound in the earpiece.

On a later visit to the dealer's shop I was able to get a similar set but in better overall condition. The metal plate on this set called it a fieldphone with the information that the handsets were sound powered (ie. not requiring batteries).

The dealer also told me that the sets had come from the local Royal New Zealand Navy Surplus Stores and, in his opinion, would have been used for emergency calls to get "all hands on deck" or similar.

A further mystery is that, with a short circuit across the output of the generator, almost all resistance when turning the handle disappears. Any students of electrical theory who can explain this?

I have not yet been able to prove over what length of cable or line the sound powered handsets might work as there is no induction coil or similar provided.

On a recent visit to one of our latest ANZAC Navy vessels we were shown emergency phones that similarily used sound powered transmitters with no battery or induction coil.

Has anyone any ideas?

Geoff Jull
Auckland, New Zealand

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