Mirrors of Magnetism. 



215 



5 centim. long and 4 ceniiin. mean diameter, the winding at 

 the ends being exposed so that it could be put up close to the 

 iron. A search-coil of 100 turns, about 1^ centim. in dia- 

 meter, was mounted so that it could be fixed at different 

 positions relatively to one of the solenoids, which was then 

 placed upon the iron plate* and upon the other solenoid 

 alternately. A current of constant value was turned on and 

 off in each position of the solenoid, and the throw on a ballistic 

 galvanometer in the circuit of the search-coil observed. The 

 following table, giving the throw of the needle for different 

 positions of the search-coil, shows how perfectly the iron 

 plate mirrored the magnetism of the solenoid. The dis- 

 crepancy in the figures arises more from errors of observation 

 than anything else. 



Position of Search-coil. 





1 a 1 



n 1 



middle i | 



1 a 1 



end | a . 



a 



□ 



1 1 



a 



1 1 



Throw when coil 

 on coil. 



Make. 



Break. 



236 



236 



235 



235 



230 



230 



227 



227 



204 



202 



129 



126 



69 



67 



41 



41 



24 



24 



Throw when coil 



on iron. 



Make. 



236 

 234 



230 



228 



204 



129 



69 



41 

 24 



Break. 



235 

 234 



229 



227 



202 

 127 



68 



41 

 23 



* 



2 feet. 



The iron plate was ordinary boiler-plate J inch thick, size 3 feet by 



