A loosely wound helix made of stiff wire is mounted verticaIly with the lower end just touching a dish of mercury. When a current from the battery is started in the coil through the mercury

When a current is passed through the helix, the neighbouring coils of the helix attract each other due to which it contracts. As a result the contact is broken and the coils will recover their original state under the influence of a restoring force. The contact is made again and the process continues. Thus, the wire oscillates.