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MANAGERS AND PROJECTIONISTS 591
gives five different values of current, and the change may be made from one point to another without breaking the arc.
THE UPPER ADAPTING LINK, 17, Plate 4, is for the purpose of changing the connections to the reactance coil, so as to provide proper voltage adjustment at the arc for different supply circuit voltages. In other words, the A. C. supply may be 220 or 110 on the face of it, whereas the actual pressure in the theatre, owing to drop in line, etc., may be anywhere between 210 and 230, or 105 and 115 volts. By means of this link it is possible to provide for these variations and make a connection suited to the actual voltage, which easily may be determined by using an A. C. voltmeter. If a voltmeter is not available the lighting company should be requested to make the test.
THE LOWER LINK CONNECTOR, 18, Plate 4, is used in emergency, to transfer the arc from the tube circuit to direct operation on the alternating current circuit, in case the tube should fail or something else happen to the rectifying side of the machine. For direct current operation (rectification) this link should be placed so as to join the lower of the three terminals and the upper right hand terminals, marked "D. C. Arc"; for alternating current operation the link should join the lower terminal and the upper left hand terminal marked "A. C. Arc." Be sure that the wing nuts are well tightened so as to clamp the links firmly.
THE TUBE is a glass vessel into which a small amount of mercury has been placed, and from which all the air has been removed, causing a vacuum. The general characteristics of its operation have been described under "General Remarks," page 515. It has four terminals, the upper ones being the graphite anodes, the smaller, lower one the starting anode and the larger lower one the cathode; both the two lower are of mercury. These various terminals are connected to coiled leads L, M, N, O, Plate 2, by means of brass spring clips, as at 9, 9, 12, 29, Plate 4.
INSTALLATION. — The rectifier will be received in two shipments. The glass tube, carefully packed in a special crate, is usually sent by express, whereas the remainder of the outfit, being the completely assembled rectifier (except the tube) all ready for operation, will probably be sent by freight. When the outfit is received, remove it from its case and place in the location selected. Remove the perforated. sheet steel cover and connect the A. C. feed wires to rectifier leads H, Plate 3, through a line switch and fuses.