A loosely wound helix made of stiff metal wire is mounted vertically with the lower end just

   touching the mercury in a dish. When the current from a battery is started in the metal wire

     through the mercury. then the spring

When lower end comes into contact then spiral wire rings get attracted to each other end get compressed and looses contact. As they become normal again current flow take pl      making it to compress again. In this way it executes oscillatory motion