CHAP, v.] ELECTRICITY AND MAGNETISM. 



283 



that of a sphere of one centimetre radius, =63 x io 7 (electro- 

 static) units. The magnetic moment of the earth is, according 

 to Gauss, no less than 85,000,000,000,000,000,000,000,000 

 times that of a magnet of unit strength and centiin. length, i.e. 

 its magnetic moment is 85 x io 24 units. The resistance of 

 selenium is about 40,000,000,000, or 4 x io 10 times as great as 

 that of copper ; that of air is about io 26 , or 



i oo, ooo, ooo, ooo, ooo, ooo, ooo, ooo, ooo 



times as great. The velocity of light is about 30,000,000,000 

 centimetres per second, or 3 x io 10 . As a final example we 

 may state that the number of atoms in the universe, as far as 

 the farthest visible star, can be shown to be certainly fewer than 

 7 x Io91 --^-** if* /o~ : 



^ & r~ , ft 



LESSON XXVI. Electromagnets. 



326. Electromagnets. In 1820, almost immedi- 

 ately after Oerstedt's discovery of the action of the 

 electric current on a magnet needle, Arago and Davy 

 independently discovered how to magnetise iron and 

 steel by causing currents of electricity to circulate round 

 them in spiral coils of wire. The method is shown in the 



Fig. 114. 



simple diagram of Fig. 114, where a current from a 

 single cell is passed through a spiral coil of wire, in the 



