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CURRENT RECTIFICATION
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permanent intimate contact with the surface. A bolt is used, the end of which may be seen in Fig. 74 E. (52 ) The precise degree of pressure of the lead washer
upon the copper oxide coating is still a problem. Too much or too little pressure interferes with the proper rectifying action.
(53) In commercial form the stack rectifier consists of a number of lead and copper washers pressed together as pictured in Fig. 74E and diagramatically illustrated in Fig. 74G. In 74G CC represents copper washers coated on one side with copper oxide, LL lead washers used to make connection with the copper oxide, BB represent the copper-oxide coating, AAA heat-radiating fins. In radiating heat to the air, fins are highly efficient. In addition there is a fan, binding post and a protective casing. (54 These rectifiers are not likely to require either repairs or replacements. In fact, they should be left entirely alone, except for careful'and frequent inspection of the electric fan that is used for cooling. (55) A poor fan will soon cause overheating of the rectifier, which will injure it seriously. However, manufacturers of the projection lamp rectifier are now using a mercoid switch. The breeze created by the fan operates upon a flexible arm to keep the mercoid switch closed. If the fan stops, or slows down the mercoid switch opens automatically, stopping all current from flowing into the rectifier units. Approved, brushless, ball-bearing fan motors are used Tor this work.
Fig. 741 1 shows a stack rectifier that is widely used.
(56) The efficiency of stack rectifiers is comparable
to the operating averages of other types of current
Fig. 74G. — C C are copper discs or washers ; B B shows the copper oxide (or sulphide) coating; L L are lead discs or washers and AAA are metal "fins" to aid in heat radiation.