Richardson's handbook of projection (1927)

580 HANDBOOK OF PROJECTION FOR as follows : When the lamp carbons are brought together the effect is, to all intents and purposes, to form a short circuit, which would have the effect of sending a heavy rush of current through the arc circuit. Resistance 3, Fig. 188, takes the place of the resistance the arc will offer after the carbons are separated. This resistance is automatically cut into circuit when the plunger of relay 4, Fig. 188, is down ; or, in other words, when relay 4 is "open." When the carbons are opened and the arc struck, the effect is to add the resistance of the arc to the resistance offered by currentlimiting resistance, 3, and thus raise the voltage of the lamp circuit. When this voltage reaches a certain point (about 40 Figure 189 i volts) the energy of the magnet of relay 4 becomes sufficient to raise plunger 5, Figs. 188 and 189, and bring blade 6, Figs. 188 and 189, into contact with block 7, Figs. 188 and 189, thus short-circuiting current-limiting resistance 3, and raising the D. C. amperage. Should relay 4 at any time fail to act, it is likely plunger 5, Figs. 188 and 189, is stuck, which might be caused by a grain of sand, a bit of dirt or from some other cause. This plunger may be removed from the magnet by pulling out split key 18, Figs. 188 and 189, and, while holding stationary nut 9 at the top of the plunger, unscrew plunger 5 by turning its lower end. Having removed the plunger and ascertained the cause of its sticking it may be replaced, and when you are able to get split key 18 into its hole the plunger is in the proper location. In replacing nut be sure to get it