38 PHYSIOLOGICAL PHYSICS. [Chap. iv. 



wire. The ends of the wire are connected to binding 

 screws, to which wires can be connected for leading off 

 the induced currents, for instance to a galvanometer, 

 as shown in the figure, which indicates their existence. 

 The coils are so made that the primary can slip out of 

 or into the interior of the secondary, which is hollow. 

 By such an arrangement all the phenomena described 

 can be readily observed. Such a coil is called an 



INDUCTION COIL. 



Induction toy a magnet. Not only a current 

 of electricity, but also a magnet, is capable of produc- 

 ing induced currents. Thus, suppose the element of 

 primary coil of Fig. 20 to be removed, and let a 

 magnet (AB, Fig. 21) be substituted 

 for them. If the magnet be held 

 stationary above the secondary coil 

 no current is induced. But as soon as 

 the magnet is thrust into the interior 

 of the coil, or removed from it, induced 

 currents are evident, in opposite direc- 

 tions, which also obey Lenz's law. 

 If the magnet be stationary and the 

 neto-ETectrfc co ^ moved, induction currents will also 

 induction. b e produced. Thus, if coils be rotated 

 before the poles of a strong magnet, a 

 large number of induced currents can be produced, 

 rapidly following one another. This is the principle 

 made use of in the construction of the magneto-electric 

 machine for medical purposes, and in machines for the 

 production of the electric light. It is found also that 

 if in the interior of the secondary coil a core of soft 

 iron be placed, and if one pole of a strong magnet be 

 then brought into contact with the core, it becomes 

 magnetised ; on removing the magnet the core loses its 

 magnetism, but this magnetisation and demagnetisa- 

 tion of the soft iron core produce induction currents in 

 tha.coil. It is further known (page 53) that a current 



