419 



and the method is quite inapplicable if both the quantity and 

 intensity be not considerable. 



By passing the current through a Breguet's metallic ther- 

 mometer. This method is much used by some electricians, 

 but, besides requiring an expensive and delicate apparatus, 

 the results of both this and the foregoing methods are affected 

 by the temperature and currents of the air. 



By the voltameter ; that is, by the measured quantity of 

 gas evolved per minute from the decomposition of water. 

 This method requires a somewhat tedious observation, and 

 is only applicable to the measurement of currents of con- 

 siderable energy. 



By the magnetic needle galvanometer ; which, perhaps, I 

 may be permitted to describe somewhat in detail. When a 

 voltaic current is passed through a conducting wire, and a 

 delicately-suspended magnetic needle is brought near it, the 

 needle will exhibit a tendency to place itself at right angles 

 to the wire. If the current be supposed to proceed from 

 east to west, and the needle to be placed above the wire, the 

 end of the needle, commonly called the north pole, will point 

 to the north. From this fact Dr. Faraday, being anxious to 

 avoid the false nomenclature of positive pole and negative 

 pole, has named the terminals of voltaic circuits anode and 

 catode, — that is, the way up and the way down ; and in rela- 

 tion to the idea of electric currents passing in the apparent 

 direction of the sun round the earth, and causing the pheno- 

 mena of terrestrial magnetism. 



The needle does not, however, place itself at right angles 

 to the conductor in obedience to repulsion, as appears at first 

 sight to be the case, but really from an inclination to revolve 

 round the conducting wire. This was pointed out by Dr. 

 Faraday amongst his earliest discoveries. 



It was soon discovered that if the conducting wire were 

 bent, so as to pass both above and below the needle, the 



H H 



