Leyden Jar is a particular form of electrical condenser (q.v.). A glass jar is coated on its lower half inside and outside with tinfoil, the lip and upper half being varnished to improve the insulation. Sometimes the jar is partly filled with strong sulphuric acid, which replaces the foil as the inner conductor, and keeps the interior surface of the glass dry by absorbing moisture. A rod terminating in a brass knob is usually fixed to a wooden stopper and connected to the inner coating. The capacity of a Leyden jar is small, but the dielectric strength of the glass is considerable, so that, by charging the jar with an influence machine until a very great difference of potential exists between the two coatings, discharges of considerable vigour may be obtained.