Mercury Arc Rectifier

By 1900 alternating current had won the argument about how to move power, and immediately created a new problem: a great many loads still wanted direct current. Tramways, electrolysis plants, battery charging and arc lamps all needed DC, and the only way to get it was a motor-generator set — an AC motor bolted to a DC generator, with bearings to wear, brushes to dress and about a tenth of the energy lost as heat and noise. Peter Cooper Hewitt’s rectifier replaced that whole rotating machine with a sealed glass bulb containing a pool of mercury and no moving parts at all. The physics it exploits is a beautiful asymmetry: a mercury pool with an arc on it emits electrons freely, while a cold graphite anode a few centimetres away emits essentially none. Current can therefore cross the gap in one direction and not the other, and the bulb rectifies. The pool being liquid is the quiet masterstroke — the cathode spot erodes its own surface constantly, and a liquid simply flows back and heals.