Page:Popular Science Monthly Volume 88.djvu/155

 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.,

^

��Poiitive

��Neutra/

��Uegaf/ve

��\y

��■fuse

��-©-

��-Q^

��•^--Jw.

��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 with 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

��c

��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.

�� �