Chap, iv.] RUHMKORFF'S COIL. 41 



commutator is described. From c the current passes 

 by the wire f to the primary coil, round which it 

 passes, and gets exit by the wire/"', which conveys it 

 to the pillar to which L is attached. The current 

 having gained L, proceeds along it to the mercury 

 cup M, and from M to the binding screw b, to which 

 the negative pole of the battery is attached, by which 

 it regains the battery. Consequently, when the 

 metallic point of L is in the mercury the current is 

 allowed to pass, when it is out of the mercury the 

 current is interrupted. Now, when the current passes 

 round the primary coil, the core of soft iron wires 

 becomes magnetised, and attracts the armature at the 

 end of L ; this raises the point of L out of the mercury, 

 and the current ceases to flow. The current having 

 ceased the soft iron core loses its magnetism, and so 

 fails longer to attract the armature. The lever, there- 

 fore, by the elasticity of the spring, is restored to 

 its former position ; the point dips again into the 

 mercury, and the current is re-established. The core 

 again becomes magnetic, again attracts the armature 

 of L, and so again interrupts the current The core is 

 again demagnetised, L springs back, and the current 

 is once more re-established. So the action goes on, 



o 



the current being automatically interrupted and re- 

 established with great rapidity, and induced currents 

 being therefore formed also with great rapidity. If 

 wires be attached to the binding screws A and B of the 



O 



secondary coil, and the ends brought near to one 

 another, but not touching, the induced electricity 

 will be seen to leap across from one wire to another 

 in a stream of blue light. This is the spark of 

 induced electricity, and in large coils a spark of many 

 inches (18 inches) may be obtained by separating the 

 ends of the wires. Thus by means of a coil having 

 a large number of turns of very fine wire, very great 

 differences of potential, and great tension, can be 



