CH. xxxvi. THERMO-ELECTRICITY. 377 



a magnet by means of an electric current, whether he might 

 riot set up an electric current by means of a magnet. 



To try this he wound from 200 to 300 yards of wire 

 round a hollow cylinder a, Fig. 68, and carried the two 

 ends of the wire to a little instrument , called a galvano- 

 meter, which was invented by Ampere, and the needle of 

 which moves directly the slighest current passes through it. 

 He then took a powerful bar magnet, ^, and held it within the 

 cylinder. The moment he put it in, the needle of the galvano- 

 meter showed that an electric current had passed through 

 the wire in one direction, and the moment he drew it out 

 another rush of electricity occurred in the other direction, 

 showing that the magnet had set up an electric current in a 

 coil of wire. While the magnet remained in the cylinder 

 there was no current ; it was only at the moment of going 

 in and coming out that it produced the effect. By another 

 piece of apparatus Faraday succeeded in making these 

 currents strong enough to produce electric sparks ; and it is 

 on this principle that the induction-coil is made which is now 

 used to produce very powerful electrical effects. 



Professor Seebeck discovers Thermo-electricity, 

 or the Production of Electricity by Hoat. The con- 

 nection was now clearly established between currents of 

 electricity and lines of magnetic action, and this gives to a 

 certain extent the answer to our question, Why does a 

 magnet turn to the north ? Ampere suggested quite early 

 in the discussion, that if an electric current will turn metals 

 into magnets, the electric currents which we know are flow- 

 ing from east to west round our globe may turn the earth 

 (which is full of metals) into a great magnet. But it is also 

 true that exactly the opposite effect is possible, and that the 

 lines of magnetic force may be started by some other cause 

 and may set up the electric currents, so that we do not 

 really know which gives rise to the other. 



