STC 4021-F "Apple & Biscuit" Microphone, Ser. No. 802, with its factory box, c.1950.

The STC 4021 series microphones were first introduced by the Standard Telephone Company and BBC in 1935. Because of the shape, they became known as the Apple and Biscuit microphone. The Romanov screen (‘biscuit’) and spherical body (‘apple’) gave this microphone a very uniform frequency response in all directions, making it very effective for recording groups of people positioned around it. The STC 4021-F was an adaptation introduced around 1950.

The 4021 was a moving coil microphone, in which a coil of wire surrounds a magnet, and is connected to a diaphragm which vibrates in response to sound waves. As the diaphragm moves, the attached coil vibrates back and forth around the magnet, generating an electrical current, which creates the audio signal.