PRACTICAL OPERATION OF DYNAMOS. 213 



various types of lightning arresters in use differ chiefly in the 

 means employed for extinguishing the arc. 



The extinguishing of an arc across the spark gaps of a lightning 

 arrester is accomplished by one or more of the following means : 



(a) By the magnetic blow-out. An electromagnet may be used 

 to extinguish the arc which persists across the spark gap of a 

 lightning arrester in the same way that it is used to blow out the 

 arc in a circuit breaker. The spark gap need only be placed 

 between the poles of a fairly strong electromagnet. The mag- 

 netic blow-out is not suitable for alternating current circuits. 



(^) By cooling the arc. An alternating current arc does not 

 persist across even a very short gap between massive blocks of 

 metal unless the electromotive force is very large. Thus an 

 alternating electromotive force of 500 volts effective value cannot 

 maintain an arc across a g 3 ^ inch gap between massive blocks of 

 brass. This is due no doubt to the cooling of the vapors of the 

 arc by the metal blocks at the time of a reversal of the alternating 

 current, when the current is nearly zero in value ; the cooled 

 vapors being a sufficiently strong insulator to withstand the suc- 

 ceeding wave of electromotive force. 



(c) By smothering the arc. An arc cannot be maintained, ex- 

 cept by an excessively high electromotive force, in a narrow 

 enclosed space. This fact is utilized in several types of lightning 

 arresters by enclosing the spark gap in a narrow space surrounded 

 by insulating material. 



(d) By using- resistance. The maintenance of an arc depends 

 upon the flow of considerable current. Hence an arc cannot be 

 maintained across the spark gaps of a lightning arrester if suffi- 

 cient resistance is connected in series with the gaps. This 

 resistance should of course be non-inductive, and it should be 

 made of bulky material so as to absorb, without excessive rise 

 of temperature, the heat generated in it by the lightning dis- 

 charge. The best form for this resistance is a solid rod of non- 

 conducting material, such as burned clay, coated with a thin layer 

 of conducting material, such as graphite. Such a coated rod 



