AN investigation into the behaviour of selenium contact rectifiers

The earliest report on contact rectification dates back to 1874 when Braun published the results he obtained from experiments with natural metallic sulphides and also with some specially prepared compounds suitably arranged between contact clamps. Branley subsequently studied the rectifying properties of copper oxide in contact with different metals. In 1904, Midgley described a detector consisting of an asymmetrical junction between a metal such as silver and a metal oxide such as silver oxide or copper oxide. In the following four years the crystal detector appeared and reports on the characteristics of crystal detectors or rectifiers were published by Dunwoody, Pearce and others. In 1908 Thompson patented the first copper oxide rectifier. This consisted of a small copper disc on the one edge of which an artificial layer of oxide was produced by the action of heat. This was pressed against a soft metal electrode of lead or tin. In the years following up to 1925 may types of contact rectifiers were described and patented The latter year saw the commercial development the contact rectifiers. The two principal developments were the cupric sulphide rectifier described by Ruben, and the copper oxide rectifier described by Grondahl and his associates. The former type is of the low resistance class and is mainly used for high current densities. This type has received scant treatment in the literature on the subject. The latter type is of the high resistance class and is better adapted to the case where the current to be passed is small.