CHAP, x.] ELECTRICITY AND MAGNETISM. 



367 



Aragtfs rotations by supposing there to be a sort of 

 magnetism of rotation, until Faraday proved them to 

 be due to induction. A 

 magnetic needle set swing- 

 ing on its pivot comes to 

 rest sooner if a copper disc 

 lies beneath it, the induced 

 currents stopping it.\ If a 

 cube or disc of good con- 

 ducting metal be set spin- 

 ning between the poles of 

 such an electromagnet as 

 that drawn in Fig. 127, 

 and the current be suddenly 

 turned on, the spinning metal 

 stops suddenly. If, by sheer 

 force, a disc be kept spin- 

 ning between the poles of 

 a powerful electromagnet it 

 will get hot in consequence 

 of the induced currents flow- 

 ing through it. In fact, 

 any conductor moved forc- 

 ibly across the lines -of- 

 force of a magnetic field 

 experiences a mechanical 

 resistance due to the in- 

 duced currents which op- 

 pose its motion. 



4O2. Induction - cur- 

 rents from Earth's Mag- 

 netism. It is easy to ob- 

 tain induced currents from the earth's magnetism. A 

 coil of fine wire joined to a long-coil galvanometer, when 

 suddenly inverted, cuts the lines -of- force of the earth's 

 magnetism, and is traversed accordingly by a current. 



Faraday, indeed, applied this method to investigate 



Fig. 150. 



