320 THE ELECTRIC TELEGRAPH. 



F (0), F (0!), E, and E x eliminated, 



E = E! 



r \ 



61. Ohm's method, although it has rendered good service 

 to the science in being the means of measurements of great 

 value by Professor Poggendorff, is less to be recommended, 

 as it depends upon the correctness of the functions of the 

 galvanometer deflections ; whereas all those methods from 

 which these functions are eliminated are to be preferred. 

 By the method invented by Professor Ohm, a galvanometer 

 and set of resistance coils are connected in circuit with the 

 elements to be measured, and the intensities observed with 

 two different values of the interposed resistance. E being 

 the electro-motive force, 1^ the resistance of the element, r t 

 and r u the two interposed resistances, and I ; and I y/ the 

 observed intensities, 



*' = BT+T and l " = R + r 

 whence 



f*. N T T 



E 



!-* 



Thus E, which is expressed in arbitrary units of resist- 

 ance and intensity, may be compared with the E measured 

 in the same way, of some constant or unit element, or I y 

 and L may be expressed in cubic centimetres of water 

 decomposed in a minute, and r and r in the units of copper 

 prism mentioned before. 



62. Compensation Method of Poggendorff. This is the 

 most elegant of all the methods yet introduced. It has, in addi- 

 tion to the advantage of comparing the electro-motive forces 

 of two elements or batteries by a single observation, that of 

 being independent of any detrimental polarisation. This 

 is one of the few null-methods which are applicable in elec- 

 tricity ; that is to say, one of those methods of measurement 

 in which we balance the currents either of two batteries or 

 of two circuits, so that we have a circuit whose electrical 

 conditions are such that, if we insert a galvanometer, no 

 traces of current can be perceived. 



