32 ROTATION EXCITES THE ELECTRIC ETHER. 



The actual contact and friction of the surfaces 

 of all bodies excite heat and light, as is familiarly 

 known; and a similar contact and friction of a 

 rubber on the surface of rotated cylinders of glass, 

 sulphur, and resinous substances, were originally 

 employed for exciting electricity, as represented by 

 Fig. 5. But the most effective kind of electri- 

 cal machine is 

 the invention 

 of Professor 

 Holtz, Fig. 

 6 ; with one 

 glass plate 

 revolving op- 

 posite to an- 



Fig.'~ other, without 



contact, and with a space of air intervening be- 

 tween them, as between the earth and the sun. 

 Rows of pointed wires are arranged in the usual 

 way to intercept and conduct away the electric 

 currents. By means of this machine, bright elec- 

 tric flashes are seen to pass nearly two feet through 

 the air, in a zigzag course. 



Another efficient mode of disturbing and put- 

 ting in motion the electric ether in currents was 

 originally devised by Professor Faraday, who had 

 recourse to the use of excited magnets to induce 

 excitation of more powerful electric action. He 

 used a spiral coil of wire to be slipped down 

 over the pole of a magnet, as shown in Fig. 7, 



