Natural Philosophy Collection

Mercury Vapour Arc Rectifier

The spectacular monster shown in the picture beside a domestic radio valve is more likely to be seen today as a prop in a horror film than as a working device. The only thing, though, that might be brought to life when connected to its electrodes would be a heavy-duty motor, such as could power a light suburban railway engine. The purpose of the device was to produce a direct current (DC) from an alternating current (AC). Monsters like ours could well provide some 1200 volts at a few hundred amps, a lethal combination of voltage and current. Part of the fascination of the piece lies in it being one of the largest vacuum tubes to have evolved, the elephant in the glass valve zoo. The design was developed in the first quarter of this century from the mercury vapour discharge lamp: a substantial pool of mercury in the base acted as a cathode, feeding the 3 side-arm anodes, one for each phase of a 3-phase supply. Three intermediate grids provided some control over the large current. In operation, the pear-shaped bulb glowed a fiendish violet-green while tears of condensed mercury ran down within. Compared with the alternative rotary AC-to-DC converter based on heavy engineering electric motor technology, the somewhat awe-inspiring mercury vapour rectifier was small, silent, vibration free, reliable and efficient.

John S. Reid