Consists of a metal tank 150 x 90 x 100 mm L x B x H has six apertures on one side through which 100 x 3 mm length x dia metal rods of copper, brass, aluminium, iron, zinc and lead pass. The outer end of each rod had a cavity. When the low melting point paraffin wax is placed in the cavity, the different metals melt the paraffin at different rates.