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In connection with some experiments involving the use of a thermo-microphone of small dimensions, I tried one of large proportions, to compare its efficiency with that of the small one. It consisted of a loop of silver gilt wire, 4 feet in length, stretched lightly between two glass insulators mounted on a stiff wooden batten. It was joined up in a circuit with a 10-volt battery and the primary winding of the step-up transformer of a three-valve (low frequency) amplifier, the secondary winding of the third valve of the amplifier leading to a pair of 60-ohm Brown telephones, which were used for recording the effects of sound vibrations on the loop of wire. The listener at the telephones was situated in a quiet place some distance from the room in which this wire was fixed, and he could not hear with his naked ears the sounds produced to test the microphonic capabilities of the warmed wire. The response to these sounds in the telephones was well marked, the wire evidently acting as an efficient, though weak, microphone. Musical notes, produced by blowing organ pipes and a small syren, were loud and clear; laughing, whistling, and humming were easily differentiated, though speech was not intelligible.

Experiments on the effect of the vibration of a stretched wire forming part of closed electric circuit