Exhibit of the Month January 2012
Mercury Vapour Rectifier, circa 1941
As we know, alternating currents (AC) come from the wall outlet. Since the end of the 19th century, AC and rotary currents have become the standards because their leakages are the lowest during transmission. For some transmissions, however, one still needs direct current (DC); for example, in the operation of streetcars, trains, and large-end systems. Also, batteries can only be charged with DC. Thus, a rectifier converts AC into DC.
A mercury vapour rectifier consists of an evacuated glass bulb, in which graphite electrodes are sealed in the upper part. In the lower part, there is a pool of mercury.
In order to work, the rectifier has to be started. For that, the mercury is locally heated to the point that particles begin to evaporate. This is usually achieved with an auxiliary electrode. Through collisions, some of the evaporated quicksilver atoms are ionized, i.e., divided into positively and negatively charged particles. This creates a flow of (negatively charged) electrons from the hot spot on the quicksilver pool to the graphite anode. This is visible as an arc. The positively charged quicksilver ions fall back on the cathode spot heating up the mercury, and new atoms evaporate keeping the process going.
A Patent from 1902
A patent for this type of rectifier was filed in 1902 by Peter Cooper Hewitt. Because of the danger that the glass flask could break, metal instead of glass was later used. Increasingly larger and more stable rectifiers emerged, which were used until 1960 for the generation of DC.
The exhibited vapour rectifier tube was probably made in 1941 by the firm Paul Hardegen GmbH, Berlin SO 33. This firm built home and office equipment such as pneumatic tube systems, document elevators, telephone systems, and air supply systems. Furthermore, they produced rectifier tubes with the brand name "ARGONAL"—the so-called Argonal vapour rectifier.