CHAP, x.] ELECTRICITY AND MAGNETISM. 365 



again by a spring when, on the breaking of the circuit, 

 the core ceases to be a magnet. A more common 

 interrupter on small coils is a "break," consisting of a 

 piece of thin steel which makes contact with a platinum 

 point, and which is drawn back by the attraction of the 

 core on the passing of a current ; and so makes and 

 breaks circuit by vibrating backwards and forwards just 

 as does the hammer of an ordinary electric bell. 



Associated with the primary circuit of a coil is usually 

 a small condenser, made of alternate layers of tinfoil and 

 paraffined paper, into which the current flows whenever 

 circuit is broken. The object of the condenser is, firstly, 

 to make the break of circuit more sudden by preventing 

 the spark of the " extra- current " (due to self-induction 

 in the primary circuit) (Art. 404) from leaping across 

 the interruptor ; and, secondly, to store up the electricity 

 of this self-induced extra-current in order that, when 

 circuit is again made, the current shall attain its full 

 strength gradually instead of suddenly, thereby causing 

 the inductive action in the secondary circuit at " make " 

 to be comparatively feeble. 



399. Ruhmkorff's Commutator. In order to 

 cut off or reverse the direction of the battery current at 

 will, Ruhmkorff invented the commutator or current- 

 reverser, shown in Fig. 149. In this instrument the 

 battery poles are connected through the ends of the 

 axis of a small ivory or ebonite cylinder to two cheeks 

 of brass V and V, which can be turned so as to place 

 them either way in contact with two vertical springs B 

 and C, which are joined to the ends of the primary coil. 

 Many other forms of commutator have been devised ; 

 one, much used as a key for telegraphic signalling, is 

 drawn in Fig. i 59. 



400. Luminous Effects of Induction Sparks. 

 The induction coil furnishes a rapid succession of sparks 

 with which all the effects of disruptive discharge may be 

 studied. These sparks differ only in degree from those 



