40 PHYSIOLOGICAL PHYSICS. [Chap, iv 



primary, from one-fifth to one-tenth millimetre thick, 

 and is very long. In some large coils the secondary 

 wire may be sixty to ninety miles long. Thus the 

 secondary coil is wound round the outside of the 

 primary coil, though carefully insulated from it. The 

 ends of the primary wires are connected with the 

 binding screws b and 6', and the ends of the 

 secondary wires to the screws A and B, which may be 

 supported on glass pillars, as shown in the figure. 

 In such an arrangement the coils are not movable. 

 An arrangement is now required for interrupting and 

 re-establishing the current in the primary coil with 

 rapidity, so as to produce the induced currents. One 

 method of doing this is by what is called Foucault's 

 contact-breaker. This consists of a small cup M con- 

 taining mercury, which is in metallic communication 

 with the binding screw b. Dipping into the mercury 

 is a metallic point connected with a lever L movable 

 on an axis at L. The other end of the lever has 

 attached to it a small piece of soft iron (armature, as 

 it is called), which just projects above the core of soft 

 iron jutting out of the centre of the primary coil. 

 Now, if this core becomes magnetic it attracts the soft 

 iron armature. This draws down one end of the 

 lever moving on the axis L, and raises the other 

 end, so that the metallic point is raised out of the 

 mercury, and therefore breaks contact with it. The 

 lever L is supported on a spring, by the elasticity 

 of which it is pulled back to its original position, 

 should the core lose its magnetism, and so contact 

 with the mercury would be again established. Now, 

 suppose the positive pole of a battery to be attached 

 to the binding screw b', by means of a wire laid in the 

 wooden support of the coil, the current goes to c, 

 which is an arrangement called a commutator, and is 

 for the purpose of reversing the direction of the cur- 

 rent, or preventing it from passing. In chapter vi. a 



