36 ELECTRIC EXCITATION BY COPPER PLATES. 



the two poles of a horse-shoe magnet, as repre- 

 sented in Fig. 9. The electric circuit is formed 

 by connecting wires, w, w, and a galvanometer is 

 used to indicate the power of the excited current 

 by the deflection of the magnetic needle. 1 Pro- 

 fessor Faraday says : " When this copper plate 



Fig' 9- 



was arranged to revolve with its plane at right 

 angles to the dipping-needle, the electric currents 

 circulating from east to west about the earth's sur- 

 face served as a substitute for those circulating 

 about the poles of the artificial magnet." 



Remarking on the results of his experiment, 

 Faraday says : " This affords an instructive con- 

 trast with the operation of a common electrical 

 machine. In the one is used a plate of the best 

 non-conducting material, and in the other the 

 most perfect conductor. In the one, insulation 

 is essential ; in the other it is fatal. In com- 



1 Faraday's Experimental Researches. 



