Maker's Name: 
Trimm Inc
Where made: 
c. 1930
21 × 16 × 3 cm

Each earpiece contains a dual pole permanent magnet assembly. The poles are wound with many turns of insulated wire less than 2/1000 inch diameter. A thin circular steel diaphragm is supported around its rim by the earpiece, with a very small gap between it and the magnetic poles. An audio signal is fed through the windings, thus modulating the permanent magnet field.The diaphragm vibrates in sympathy with the changing magnetic field.

The permanent magnet poles bias the diaphragm magnetically. This prevents frequency doubling of the applied audio signal.

The closer the diaphragm is to the poles, the more sensitive the earphones are to an applied audio signal. Such earphones were used for domestic 'crystal set' wireless receivers as well as in communications. The 'high impedance' meant that only very little current was drawn from the source.