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1945 Theatre Catalog, 4th Edition, Page 315 (291)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 315
Page 315

1945 Theatre Catalog, 4th Edition, Page 315

Provider of D. C. Current: The Rectifier

Discussion of the Copper-Oxide Rectifier As a Means of Converting Theatre Current

A score of years has passed since the discovery by Dr. Lars Olai Grondahl, of the Union Switch and Signal Company. that large areas of copper coated with cuprous oxide could be made to change alternating into direct current. The development of a rectifier with large contact area came at an opportune time, resulting in an immediate large business from the need for such rectifiers as trickle chargers for the radios of the early twenties.

In the past twenty years, the rectifier has come a long way. Those closely connected with it feel that it has by no means reached its limits, but that future developments will broaden its field far beyond what can be seen now. Even if no further gain be made, however, the copper-oxide rectifier still remains an exceedingly useful tool in many varied forms. Certainly there is yet no evidence of any tapering off in the rate of new applications for the copper-oxide rectifier.

In passing, it is interesting to note that no theory which explains all the known facts has yet been found for the "whyii of rectification by copper oxide.


Copper-oxide rectifiers, which, 10 or 15 years ago, were limited in size to 3 or 4 watts and used for charging radio batteries, have gradually increased in capacity to sizes up to 40 kw. The motionpicture industry has used copper-oxide rectifiers ranging from a fraction of a watt for metering purposes to several kilowatts for carbon are power supply.

Mechanical Construction

The copper-oxide rectifier is fundamentally a very simple device, consisting of three elements: (1) a good conductor (copper), (2) a poor conductor or semiconductor (red cuprous oxide), and (3), an insulating layer, known as the barrier layer, located between the two large conductors. (The barrier layer has never been actually located or identified, but its existence and dimensions have been determined by measurements of electrostatic capacity and other careful experimental work. The cuprous oxide is formed from the copper at temperatures approaching the melting point of copper.

To provide a suitable low-resistance contact to the oxide, one of two assemblies may be used.

The pressure type or washer type consists of copper-oxide washers, terminals, lead washers against both surfaces of the copper-oxide washer, and two heavy washers to provide a uniform high pressure over the entire surface. The function of the lead washer is to take up irregularities in the surface of the copper 1945-THEATRE CATALOG

oxide washer, thus insuring against damage from excessive localized pressure. It also serves as a current collecting means. The pressure-type assemblies range in capacity from .015 watt for the small metering rectifier to 27 watts for the largest design using 35/3-inchsquare cooling fins. As the current capacity is increased, the rectifying area must also be increased. While this may be accomplished by paralleling a number of washers, it is more practical for current ratings of 15 amperes, or above, to use the larger plate.

Since it is almost impossible to apply a uniform high pressure over a large area, and since the problem of cooling for large pressure type assemblies becomes very difiicult, a type of construction requiring no pressure was developed. This type has been designated as the

plate-type rectifier. The lead Washer used in the pressure type construction is replaced by a metal coating applied to the oxide either by spraying molten metal or by electro-plating.

Single rectifier plates are usually oxidized on both sides, but for some applications more satisfactory results are obtained if only a portion of the surface is utilized for rectidcation. The balance of the oxide surface then serves" as a heat-dissipating medium.

Electrical Characteristics

When copper oxide washers and plates are properly processed, the resistance to current fiow from copper to cuprous 0xide, commonly known as reverse resistance, is relatively high and from cuprous oxide to copper, commonly known as for TYPICAL OF THE COPPER OXIDE RECTIFIERS described in this article is the Westinghouse Electric Corporation's Rectox (photo courtesy of the Radio Corporation of America), suppIied to the theatre trade through authorized RCA theatre-equipment distributors. The rectifier is a very simple device, consisting of a good conductor, one not so good, and a barrier (insulating) layer. While it is known how it works, the /'why" has never been explained.

1945 Theatre Catalog, 4th Edition, Page 315