Mounted on a base of timber, with a glass fronted wooden cover, is a steel assembly holding the tuning fork and two movable electromagnets, one between the two prongs at the end of the fork, and the other closer to the fork base with coils on either side of the lower prong.
This device was used to maintain the vibration of the tuning fork for long periods of time, in experiments where a constant and well defined pitch was required, e.g. Doppler effect. Essentially, the apparatus functions by attaching a battery across one of the fork prongs. The current thus produced, causes the electromagnet to do work on the prong when it is moving away. The circuit is broken when the prong is moving back towards the magnet so that this advantage is not lost. This interrupted current is usually achieved by using a rigid wire contact between the prong and one terminal of the voltage source.
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