CH. Ill] ARC LAMPS WITH SMALL CURRENTS 89 



133. Turning off the small arc lamp. The snap or key switch 

 in the usual incandescent lamp socket is designed to break the cir- 

 cuit where, at most, two amperes are used. These key switches, if 

 used to interrupt a relatively large current, like that used for the 

 small arc lamp, are liable to start an arc within the socket. If such 

 an arc is started, the socket will be short circuited, resulting either 

 in the burning out of a fuse, the burning out of the socket or some- 

 thing more serious. 



The liability of a socket to arc is much greater with direct than 

 with alternating current. The liability to arc is also much greater 

 if the key switch is turned slowly than when it is turned quickly. 



By observing the following directions the current may be turned 

 off with perfect safety : 



(1) Turn off the current by separating the carbons until the 

 lamp goes out, then the key switch may be used, or a plug or exten- 

 sion pulled apart. 



(2) Turn off the current by pulling the separable plug or the 

 separable extension apart (fig. 49-50). 



(3) Make use of a knife- or snap-switch (fig. 1,2, 40). 



(4) Do not turn off the current by the key switch in the bulb 

 socket. When the lamp is out, it is safe to turn the key switch in 

 the socket. 



(5) Do not unscrew a plug to turn off the light, for the 

 break in the circuit is so slow that an arc will almost certainly 

 be formed. 



134. What to do in case the key switch is used and an arc 

 is formed in the socket: 



(1) Turn the key on again as quickly as possible. 



(2) If the arc lamp is still burning after turning on the key 

 switch, turn the lamp off by method i to 3 ( 133). 



(3) Go to the nearest room switch and turn off the current. 



In case a fuse is blown out which is almost sure to occur if an 

 arc is formed in the socket or if the lamp socket is burned out, it 

 is wise to call in an electrician to make the necessary repairs. 

 This, of course, assumes that the user has not the technical knowl- 

 edge necessary to make the corrections himself. It is further 



