Popular Science Monthly 



Ground Detector for Three 

 Wire Circuit 



NEARLY everyone is familiar with 

 the method of connecting a couple 

 of incandescent lamps whereby they 

 will indicate the presence of grounds 

 on a two-wire system. For such serv- 

 ice the two lamps are connected in se- 

 ries between two of the wires of oppo- 

 site polarity of the two-wire system, 

 and a ground wire is tapped between 



3 wire ma/ns., 



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Poiitive 



Neutra/ 



Uegaf/ve 



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■fuse 



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Fig. 1. Wiring diagram of a three- 

 circuit ground detector 



the two lamps. Where a ground occurs 

 on the circuit, the lamp connected to 

 the wire on which there is a ground 

 will grow dim or will go out altogether, 

 and the other lamp will burn above 

 normal brilliancy. 



The method of connecting incandes- 

 cent lamps to indicate grounds on a 

 three-wire system is not, apparently, 

 very well known. It is, however, sim- 

 ple in arrangement and operation, as 

 indicated in Fig. 1, and described in the 

 following paragraph. 



The three lamps. A, B and C, arc 

 connected between the neutral and the 

 negative wires or between the neutral 

 and the positive wire, as shown. Each 

 of the lamps should be designed for 

 the voltage between cither of the out- 

 side wires and the neutral. For exam- 

 ple, the voltage between any outside 

 wire and the neutral is 125. Conse- 

 quently A, B and C should each be a 

 125-volt incandescent lamp. 



The three lamps connected in series 

 should be protected with a fuse at each 

 tap as shown in the figure. A lead be- 

 tween lamps ./ and B witli a switch in 

 scries should be connected to the earth. 

 With the three-wire system free from 

 grounds all three lamps, A. B and (', 



127 



will burn dimly, whether the ground 

 switch GS is open or closed. If, how- 

 ever, an accidental ground occurs on 

 the positive wire, all of the lamps will 

 burn with full brilliancy if GS is closed. 

 If a ground occurs on the negative 

 wire and GS is closed, lamps B and G 

 will not illuminate, but A will burn at 

 full brilliancy. If a ground occurs in 

 the neutral wire, GS being closed, A 

 will not burn, but B and C will burn 

 dimly. The switch should always be 

 connected between the lamps {A in this 

 case) which connects to the neutral 

 wire and the next adjacent lamp. If it 

 were connected between B and C, in 

 case of a ground on the positive wire, 

 lamp C would have double voltage (250 

 volts) impressed on it, and hence would 

 quickly burn out. 



Ingenious Circuit Saves Money 

 in Photoplay Houses 



MOTION picture theatregoers de- 

 mand that one film shall follow 

 another without interruption. This has 

 given rise to a trotibling problem. Dis- 

 solving the beginning of one reel into the 

 end of the preceding one, so that a con- 

 tinuous flow of the screen narrative is 

 given, necessitates the use of two pro- 

 jecting machines, one of which is started 

 just before the other stops. This man- 

 euver requires two arcs burning at the 

 same time, and two arcs, where alternat- 

 ing current only is availal^le, means that 



/i.C^ Line 



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Rccf/f/er' 



Mochine i 



vn 



If 



Compensahr 



Moc/i/ne z 



This circuit allows one rectifier to 

 serve two motion picture pro- 

 jectors at the same time 



two alternating current rectifiers are 

 necessary. 



Motion picture operators in Philadel- 

 phia have solved the problem of supply- 

 ing current to two arcs with one recti- 

 fier by the use of a four poled double 

 throw switch connected as the accom- 

 pan\ing diagram illustrates. 



