Voigt condenser microphone, probably made by Voigt Patents Limited, Sydenham, London, England, 1920-1940.

This type of condenser microphone was known as a 'slack diaphragm' microphone. An oval central pillar with a condensing surface was covered in a thin film of insulating material, followed by an extremely thin (only one thousandth of an inch thick) sheet of metal. It was used for a period by the BBC, but was not as sensitive as other types of microphone. There was also a need to keep this type of microphone very dry which, in combination with the poor sensitivity of the device, meant the slack diaphragm microphone was less popular than other types.