In the early days of wireless a coherer was used to produce a direct current from a radio-frequency signal. The wheel coherer was developed by Sir Oliver Lodge in 1902. A sharp edged steel wheel about 1 cm in diameter dips into a small cup of mercury covered with a thin layer of oil and a d.c. bias is applied by a battery. The r.f. signal breaks down the insulating oil film allowing a current to flow between the disc and the mercury. As the wheel rotates the oil film is continuously restored so that current only flows while an r.f. signal is present. This instrument is a fairly late model from a time when such coherers were in regular use for wireless telegraphy. The large brass box contains a spring-driven clockwork mechanism wound by the large handle. The wheel is at top right and can be seen in image P0266b.

Inscriptions / Translations:  B’ham. Univ. Physics E457