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294 RICHARDSON'S BLUEBOOK OF PROJECTION
their first 3,000 hours of life. After that use, their output declines slightly, and thereafter remains constant through an indefinite future. Aging tests that have now been in progress for seven or eight years give no definite indication that copper oxide units ever wear out. The taps provided on the transformer secondaries allow readjustment of the transformer output to the copper oxide units when the period of aging is completed, and thereafter need no further adjustment.
The six rectifier stacks are drawn below the transformer secondaries.
Tracing the circuit of the left-hand secondary at the moment when its left-hand end is positive: that circuit runs down, right, down, right through the top righthand stack, and down, left and down to the arc. The negative return from the arc is up, left, up to the middle left-hand stack, and up to the right-hand end of the same secondary. When the polarity in that secondary is reversed, the circuit from positive to negative is : down from the right-hand end and left through the middle right-hand stack, down, left and down to the arc : up, left and up to the top left-hand stack, through this and up to the left-hand end of the same secondary.
The circuits of the other two phases can be traced similarly, and it will be found that a full-wave rectifier is connected across each secondary of the transformer, the d.c. outputs being paralleled.
Double or "twin" copper-oxide rectifiers are also commercially available, similar to Fig. 741 except that the transformer has two separate sets of secondaries, each supplying its own independent set of six stack units. Each set of six stack units then supplies its own projector.
Such double rectifiers are sometimes provided with external switches, permitting operation of both projectors from either set of stacks, in the unlikely event of trouble with the other.
Installation — Operating Instructions
(59) Since excessive heat is injurious to the units,