Richardson's handbook of projection (1927)

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578 HANDBOOK OF PROJECTION FOR Xw* carbons have been separated so that the voltage between them is about 45, the potential relay 4, Fig. 188 (if it is a 40 or 50 ampere rectifier ; there is none on the smaller size) will operate and short-circuit current limiting resistance 3. Fig. 188, thus increasing the arc current to whatever value the dial switch is set for. CAUTION. — When you first begin to use a rectifier be sure that the potential relay operates. If it does not, current limiting resistance, 3, Fig. 188, will heat, and while it would be difficult to actually burn it out, damage might be done to it or to the insulation of surrounding wires. The projectionist can tell when this relay acts, as follows: When the carbons are first separated the current will be comparatively weak, but when the relay acts there will be a sudden increase in brilliancy at the spot. The knack of detecting the acting of the relay can be acquired by starting the arc several times and slowly separating the carbons until the relay picks up, having a man at the rectifier to tell you when it does pick up in case the rectifier is at a distance. To stop the rectifier, open the projector table switch, though opening either the switch on the A. C. lines or the triple-pole switch in the face of the rectifier panel will have the same result. OPERATING TWO ARCS FROM ONE RECTIFIER.— When it is desirable to operate two arcs from one rectifier the General Electric Company will furnish two resistances equipped with contactors, one to be used in series with each lamp. These resistances consist of several tubes, capable of being mounted on pipe or flat surface. Diagram, Fig. 188a shows the resistance connected in the lamp circuits. The operation of dissolving one reel into another is briefly as follows : Assume the projectionist to be running a picture on the left * ^i f?*S/.sr*Nce WAX H •£««•*•*• Figure 188a