British Thomson-Houston (BTH) Carbon

The BTH carbon microphone is briefly mentioned in Amateur Wireless magazine in November 1932, in a wider article about microphones at the BBC:

"A small stable microphone which is in some of the news studios and studio silence cabinets is the B.T.H. carbon. This is a fairly directional microphone, giving a crisp reproduction very suitable for speech. It has a comparatively silent background, considering its sensitivity."

The ORBEM website has a page about the BT-H carbon microphone, which show a similar model in use at the BBC in 1932.

BTH company history, from Wikipedia

"British Thomson-Houston (BTH) was a British engineering and heavy industrial company, based at Rugby, Warwickshire, England. Originally founded to sell products from the Thomson-Houston Electric Company, it soon became a manufacturer using licences from the American company. They were known primarily for their electrical systems and steam turbines.

BTH merged with the Metropolitan-Vickers company in 1928 to form Associated Electrical Industries (AEI), but the two brand identities were maintained until 1960. The holding company, AEI, was bought by GEC in 1967.

The British Science Museum have a BTH carbon mic that they date to 1922. Unfortunately there are no photographs or documentation to support that date.

Our example has a measured output impedance of around 500 ohms, after 90 years of use. It is a little noisy but has a strong output and a very smooth bass response for a microphone of this type.

It has a bakelite body and front and rear chrome plated grills, and clearly would have been an expensive device. It has four sturdy lugs which show that it was designed to be mounted with springs in a hoop. However, there is also a 1/4" threaded hole so that it can be screwed directly onto a microphone stand.

Cinephonics have one to rent

Northwestern university have a photograph in their collecttion of this microphone in use at a Nigerian radio station, taken by E. H. Duckworth.

The British Science Museum have two BT-H carbon microphones in their collection, albeit with little additional information. Example 1. Example 2.