Barker.] ^^0 [jan. 19, 



the earth exerts on a gram weigh t, there exists between them a difference 

 of potential of 980 absolute units. By measuring the force between two 

 electrified bodies in grams, the difference of potential or the electromotive 

 force between them is easily calculated in absolute measure. By mul- 

 tiplying this value in electrostatic units, by thirty thousand million, the 

 electromotive force is obtained in absolute electromagnetic units. 



The instrument used for measuring differences of potential is called an 

 electrometer ; if by direct measurement, an absolute electrometer. The ab- 

 solute electrometer of Sir William Thomson is the best thus far devised. 

 This instrument consists of two metal plates, one of which, the smaller, 

 is provided with a guard ring so that the electrical distribution shall be 

 uniform ; these plates being so arranged tliat the attraction between them 

 may be very accurately measured. The force may be measured at a con- 

 stant distance by varying the weight necessary to balance it ; or the dis- 

 tance may be varied until the force balances a constant weight. The latter 

 method is preferred in the absolute electrometer of Thomson. With this 

 instrument, the electromotive force of a Daniell cell was found to b& 

 0.00374 electrostatic unit, corresponding to 112 million electromagnetic 

 units. 



Relative measurement ot electromotive force, especially for practical 

 purposes, is much more frequent than absolute measurement. Although 

 the same units may be used, yst in practice it has been found more con- 

 Tenient to employ a separate unit called the volt, the value of which is 

 given as one hundred million absolute electromagnetic units. Moreover, 

 this unit is represented not in the abstract form alone, but also concrete. 

 Borne distinct electromotor, the difference of potential between the elec- 

 trodes of which has been accurately measured, is taken as the standard. 

 For example, the Daniell cell above mentioned has an electromotive force, 

 by the definitions already given, of 1,13 volts. Such a battery, used for 

 measurement, is called a standard battery. 



For determining an unknown electromotive force, it is only necessary 

 to determine the ratio between this and the electromotive force of the 

 standard battery. Two general methods of doing this are in use ; the one 

 direct, the other indirect. In the direct method, an electrometer which 

 has been calibrated is employed ; i. e., one whose constants have been de- 

 termined by comparison either with the standard battery or with an abso- 

 lute instrument. Such are the portable and the quadrant electrometers of 

 Thomson. In the latter instrument an 8-shaped needle of aluminum 

 swings in a cylindrical metal box with separated quadrants. The alternate 

 quadrants are electrically connected when the instrument is in use. A 

 small charge being communicated to the needle — previously adjusted sa 

 that its axis is parallel to the line between adjacent quadrants— any electri- 

 fication of the quadrants is made apparent by the motion of the needle ta 

 the right or left. By connecting these quadrants, first with the electrodes 

 of the standard cell, and then with the cell whose electromotive force is to- 

 be measured, the ratio of the deflections gives the ratio of the electromo- 



