Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/456

may be made equal to that of the electromotor $E_{1}$. If the electrodes of this electromotor are now connected with the points $A_{1},\,B_{1}$ no current will flow through the electromotor. By placing a galvanometer $G_{l}$ in the circuit of the electromotor $E_{1}$, and adjusting the resistance between $A_{1}$ and $B_{1}$, till the galvanometer $G_{1}$ indicates no current, we obtain the equation

$E_{1}=R_{1}C$,

where $R_{1}$ is the resistance between $A_{1}$ and $B_{1}$. and $C$ is the strength of the current in the primary circuit.

In the same way, by taking a second electromotor $E_{2}$ and placing its electrodes at $A_{2}$ and $B_{2}$, so that no current is indicated by the galvanometer $G_{2}$,

$E_{2}=R_{2}C$,

where $R_{2}$ is the resistance between $A_{2}$ and $B_{2}$. If the observations of the galvanometers $G_{l}$ and $G_{2}$ are simultaneous, the value of $C$, the current in the primary circuit, is the same in both equations, and we find

$E_{1}:E_{2}::R_{1}:R_{2}$.

In this way the electromotive force of two electromotors may be compared. The absolute electromotive force of an electromotor may be measured either electrostatically by means of the electrometer, or electromagnetically by means of an absolute galvanometer.

This method, in which, at the time of the comparison, there is no current through either of the electromotors, is a modification of Poggendorff's method, and is due to Mr. Latimer Clark, who has deduced the following values of electromotive forces:

					Concentrated solution of		Volts.
Daniell	I.	Amalgamated Zinc	HSO4 +	4 aq.	Cu SO 4	Copper	= 1.079
	II.	"	HSO4 +	12 aq.	Cu SO4	Copper	= 0.978
	III.	"	HSO4 +	12 aq.	Cu NO6	Copper	= 1.00
Bunsen	I.	"	"	"	H NO6	Carbon	= 1.964
	II.	"	"	"	sp. g. 1.38	Carbon	= 1.888
Grove		"	HSO4 +	4 aq.	H NO6	Platinum	= 1.956
A Volt is an electromotive force equal to 100,000,000 units of the centimetre-gramme-second system.