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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment

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Old 23rd Jul 2005, 1:14 am   #1
adibrook
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Default field telephones

I had (for a brief period of time) an old military field phone.

There's something that really confuses me about the design of it though.

It has two contacts for a cable to connect to other phones. The hand cranked generator provides the high voltage ac to energise the bell on the other side, but what are the two 1.5v batteries for?

I dont think the phone had any active components. Certainly no valves that I know of (and anyway...not many would run at 3v) and I think it's too old for transistors. Everything seemed to be made of passive components.

Does anyone know why a phone like that would need a 3v supply? If it has no on-board amplificaton...then why the batteries?

Thanks.
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Old 23rd Jul 2005, 8:04 am   #2
Sean Williams
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Default Re: Old telephones

Hi Adi,

The Type F Field Phone uses the 3 volt supply to act as the "carrier" for the audio - the microphones in the sets are filled with carbon granules, and they need a small voltage to wake them up, so without the voltage no current or audio will flow.

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Sean
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Old 23rd Jul 2005, 8:20 am   #3
SPCh
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Default Re: Old telephones

Okay, Adi, here goes.

The batteries are there to provide the "loop current" to make the phone work.

Imagine the batteries in series with the microphone and earpiece - so a simple series loop.

The microphone consists of loosely packed carbon granules in a container behind a diaphragm. As you speak, the vibrations deform the diaphragm which compresses and relaxes the carbon granules such that path resistance across the microphone changes.

The varying resistance modulates the current in the loop, and stimulates the earpiece.

It really is as simple as that - and very easy to demonstrate: you could make a microphone by filling a matchbox with some crushed up coke (as in coal not the other thing !) and poking a wire into opposite ends of the drawer.

The clever bit in telephony is to get the "return voice" over the same loop, but how you do that doesn't affect the above description.

The 3 volts for your field telephone is about the bottom limit. The public telephone systems around the world use 48 or 50 volts, which drops to about 5 volts at the telephone when its off hook. The standard loop current is 20mA.
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