448 On a Voltaic Standard of Electromotive Force. [June 20, 



The uses of this standard element to practical electricians are suffi- 

 ciently obvious. It may be used for determining the electromotive force of 

 other elements by the use of an electrometer or by the discharge from a 

 condenser. Or a condenser having a capacity of -~ 7 farad charged by the 

 standard cell would contain the British-Association unit quantity of elec- 

 tricity (one veber), or of the absolute unit of quantity. 



It is also of great value for maintaining a current of known strength in 

 any circuit for the purposes of experimental research. 



1.4-5-7 OHMS. 



Thus, if it be desired to produce in any circuit, a b, a current equal to 

 the British- Association current ( T hj absolute unit), it is only necessary 

 to insert in the circuit a wire (R) having a resistance of 1*457 ohm, and 

 to connect to each end of this wire the poles of a standard cell (c) with a 

 galvanometer (g), and to vary the strength of the current in a b until no 

 deflection is produced on the galvanometer; the current through a b will 

 then be equal to one British- Association unit of current, or one veber per 

 second, whatever its length or resistance. 



By varying the resistance of R, or by varying the number of elements 

 (c), any given current can be steadily maintained through a b at pleasure ; 

 on the other hand, the value of any given current can be measured by so 

 varying the resistance R that no deflection is produced on the galva- 

 nometer. The value of the passing current will then be 



C= * veber per second. 

 R 



It is also evident that, knowing the value of E, we may determine the 

 horizontal intensity of the earth's magnetism (H) in anyplace quickly and 

 simply by means of an ordinary sine or tangent galvanometer. 



In fact, the standard of electee potential is second only in importance 

 to that of^the standard of electric resistance ; and the use of such a standard, 

 combined with an auxiliary battery in the manner above described, admits 

 of a variety of applications which it is believed will be found of great 

 value in electrical research. 



jAS kd 1873 



