Carbon microphone used in association with a headphone (not present) to form a valveless amplifier. It was probably manufactured by New Wilson Electrical Manufacturing Co. Ltd in 1927.
The bar replaces the diaphragm of a headphone earpiece so the bar now vibrates instead of the diaphragm. The earpiece is connected to the output of the radio (usually a crystal set). The movement of the microphone bar changes the resistance between the microphone terminals (ie the resistance between the screw and the carbon). A pair of headphones (for listening) is connected via the microphone terminals to a battery.
A control knob allows adjustment of the sensitivity of the microphone part.
A rectangular metal shaped object with a black disc attached to one side with a screw. There is one terminal on the body and another on the black disc. Internally there is a carbon block mounted between two nickel plated ends joined by a blunt ended oval shaped piece of magnetic material. The screw, on the opposite side, can be adjusted to optimise the pressure between the carbon and the contact for maximum sensitivity.
On metal body: " MAGNETIC / MICROPHONE / BAR ", " PATENT No / 248581-25 "
Type of item
25 mm (Width), 20 mm (Depth), 55 mm (Height)
Radiomuseum [Link 1] accessed 22 Nov 2011