10 NOMOS. 



In considering this identity, we will take the evo- 

 lution of heat, the magnetism, the chemical action, 

 the shock, and the spark all of which are familiar 

 phenomena of voltaic electricity as preliminary 

 points of comparison, 



It is well known that ordinary electricity agrees 

 with voltaic electricity in the evolution of heat, and 

 that a wire may be fused by either kind indiffer- 

 ently, provided the quantity be sufficient. 



Ordinary electricity agrees with voltaic electricity 

 in the power of magnetising iron or steel, and the 

 direction of the magnetic current thus induced holds 

 the same constant relation to the electrical current 

 in either case ; but ordinary electricity has not the 

 same power of deflecting the magnetic needle as 

 voltaic electricity. M. Colladon, of Geneva, how- 

 ever, was led to suppose that this difference might 

 be owing to the use of very insufficient quantities 

 of ordinary electricity ; and, on remedying this de- 

 ficiency, he procured the wanting deflection. Nor 

 need it be any matter of surprise that deflection 

 should be wanting under ordinary circumstances, for 

 the very shock of the discharge is sufficient to de- 

 range, diminish, or even invert, the magnetic power 

 of the needle. And this is the great reason why the 

 deflection was so long wanting, for Dr. Faraday has 

 shown that the needle is deflected even by small 

 quantities of ordinary electricity, if a sufficiently 

 delicate needle be used, and if the shock be dimi- 

 nished and the discharge retarded by causing the 

 current to pass through wet thread or through some 

 other bad conductor. He has also shown that the 



