behind each of these stands a similar plate as at P, held at its centre by an adjusting screw. The outside row of plates is electrically connected with each other and with the battery, which goes to the line.

When the inside row of plates vibrates under the influence of a sound, the distance between the plates varies and changes their static capacity.

In Fig. 21 the plates are arranged as in the ordinary form of a condenser. An initial pressure is put upon them by a screw passing