240 LECTURES TO SCIENCE TEACHERS. 



pound interest principle," involved in the action of the 

 Doubler. 



I have just shown you that the deflections observed by 

 the quadrant electrometer are in simple proportion to 

 differences of potentials between the quadrants, or differ- 

 ences of potentials between any two conductors tested. 

 From these scale readings we must deduce numbers giving 

 the differences of potentials in absolute measure. One way 

 of doing this is to compare the indications of the quadrant 

 electrometer with those of an absolute electrometer, such as 

 I am about to describe immediately, and to deduce " the 

 constant " for the quadrant electrometer. The scale read- 

 ings may then be reduced to absolute measure by multiplying 

 by the constant ; and the constant of the instrument is the 

 same so long as the gauge is kept in the same condition, and 

 the quadrants in the same position as when the comparison 

 with the absolute electrometer was made. Another method, 

 which can at any time be applied with the greatest ease, is to 

 find the deflection given by a galvanic cell of known electro- 

 motive force, and deduce the constant in that way.- For 

 example, let a Daniell's cell be applied to the quadrant electro- 

 meter, and let us suppose that the deflection is seventy-five 

 S3ale divisions on one side of the middle position, and when 

 the electrodes of the cell applied to the electrodes of the 

 electrometer are reversed, a deflection of seventy-five divi- 

 sions on the other side of the middle is observed. Now the 

 electromotive force of a Daniell's cell, or the difference of 

 potentials producible by a Daniell's cell, is well known from 

 the experiments of Sir William Thomson. 1 It is 1'12 of the 

 unit termed by practical electricians a volt.' 2 ' Hence, divid- 

 ing yly^, we get 67 as the deflection produced on the 

 electrometer by a difference of potentials of one volt. 

 A double deflection would be produced by a difference of 

 potentials equal to two volts, and so on. 



[Some of the applications of the quadrant electrometer 

 were next very briefly referred to, particularly its application 

 to observation of atmospheric electricity.] For this purpose 

 it is admirably suited. The mirror electrometer is the 



1 Proc. Roy. Soc. 1860, and Reprinted Papers, xviii. 



2 One volt is equal to 10 8 centimetre-gramme-second electro-mag- 

 netic units. See Everett's Illustrations of the Centimetre Gramme- 

 Second System of Units, or F. Jenkin's Electricity and Magnetism. 



