1872.] Voltaic Standard of Electromotive Force. 



447 



We have therefore the mean value of the electromotive force 



of the standard- cells, as determined by the electrodynamo- Volt, 



meter, 18 observations 1 '45735 



As determined by the sine galvanometer, 13 observations. . 1*45621 



Mean value 1 *45 6 78 



Or, since no importance can be attached to the figures beyond the third 

 place of decimals, 1*457 volt or British-Association unit of electromotive 

 force, equal to 145700 absolute electromagnetic units. 



The value of H, the horizontal component of the earth's magnetic in- 

 tensity, a knowledge of which is necessary for the determination by the 

 sine galvanometer, was kindly supplied for each day by the Astronomer 

 Royal. 



A novel feature in both these series of determinations is the use of an 

 arrangement by which the cells under comparison are not allowed to per- 

 form any work or produce any current. 



The arrangement is shown in the subjoined diagram. 



a and b are the ter- 

 minals of the instru- 

 ment ; the standard cell 

 (C Z) is connected to 

 these terminals with an 

 intervening galv$ no- 

 meter, g ; an auxiliary 

 battery (c z) is also 

 connected with similar 

 poles to the same ter- 

 minals, so that both 

 tend to send a current 

 through the instrument 

 in the same direction. 



The strength of the 

 auxiliary battery is, 

 however, so regulated, 

 by means of the rheo- 

 stat (?•) and by varying 

 the number of cells, 

 that it just balances the 

 force of the standard 

 cell, so that no current flows through the galvanometer (g), or, in other 

 words, the terminals {a, b) are kept at a difference of potential equal to the 

 electromotive force of the standard (C Z), the current which flows through 

 the instrument being entirely supplied by the auxiliary battery. 



This method has also the advantage of being quite independent of the 

 resistance of the standard cell. 



