SU SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII 



If it is difficult to get double cord of the right size, then each of 

 the wires to the lamp can be composed of the double cable. This 

 is easily done by removing the insulation at each end of the double 

 cord and twisting both the wires together. (See the tables 694, 

 695, for the carrying capacity of flexible cord and cables). 



710. Wiring the arc lamp with a three-wire supply. Only 

 two wires go to the arc lamp, then if one must connect the arc lamp 

 for projection to a three-wire supply system it is necessary to 

 remember that the middle (neutral) wire and either outer wire will 

 give 1 10 volts the same as the two-wire no volt circuit. 



If connection is made with the two outer wires then 220 volts 

 will be used in the arc lamp. In this case a rheostat for a 220 volt 

 circuit must be employed, or two no volt rheostats in series (fig. 

 287). 



Naturally one would connect with the middle or neutral and an 

 outside wire and employ the usual no volt rheostat but for the 

 fact that such an arrangement would badly unbalance the work of 

 the line, and might cause trouble if the electric circuit was running 

 nearly on full load. It is therefore safer to connect with the out- 

 side wires and use the requisite amount of ballast. 



SWITCHES, CIRCUIT BREAKERS AND FUSES; THEIR CHARACTER, 



INSTALLATION AND USE 



711. A switch is a device by means of which a gap (fig. 275 

 and 276) can be made in an electric circuit thus stopping the flow 



of current. 



A switch should be so con- 

 structed that when it is opened 

 \ it makes a gap in all the wires 

 B of the circuit. For example, in 

 J a two-wire circuit, the switch 

 should make a gap in both wires, 



FIG. 275. CIRCUIT WITH A BREAK and in a three-wire circuit, a gap 

 OR GAP. 11 .-, rf 



.. . it in all three wires. If such a 

 Unless the metalhe circuit, irom the 



dynamo, G, back to the dynamo, is switch IS used the line beyond 

 complete, no current will flow. A gap the sw i tc h is "dead, "and no CUr- 

 m the circuit (B) prevents the now ot 

 current. rent can be drawn from it. 



