238 Common Science 



snapping all the way around and quickly leaving a big 

 gap, it moves only a little way around and an arc is 

 formed in the socket; if you hear a sizzling sound in 

 a socket, you may be pretty sure that an arc has been 

 formed. But when you pull the plug entirely out of the 

 iron or stove, the gap is too big for an arc to form and 

 you are perfectly safe. 



Fire commissions usually condemn extension lights, 

 because if the insulation wears out on a lamp cord so 

 that the two wires can come in contact, a dangerous 

 arc may easily form. And the insulation might suddenly 

 be scraped off by something heavy moving across the 

 cord. This can happen whether the light at the end of 

 the cord is turned on or off. So it is best if you have 

 an extension light always to turn it off at the socket 

 from which the cord leads, not at the lamp itself. Many 

 people do not do this, and go for years without having 

 a fire. But so might you live for years with a stick of 

 dynamite in your bureau drawer and never have an 

 explosion. Still, it is not wise to keep dynamite in your 

 bureau. 



Arc lights themselves, of course, are no more danger- 

 ous than is a fire in a kitchen stove. For an arc light is 

 placed in such a way that nothing can well come near 

 it to catch fire. The danger from the electric arc is 

 like the danger from gasoline spilled and matches dropped 

 where you are not expecting them, so that you are not 

 protected against them. 



Fortunately ordinary batteries have not enough 

 voltage to cause dangerous arcs. So you do not have 

 to be as careful in wiring for electric bells and telegraph 



