170 Dr. J. Kerr on Measurements 



sator. The quantity V is measured, with sufficient accuracy, 

 by the scale-reading of the electrometer. 

 It is already proved that, when D is given, 



Q ocV 2 ; 

 and from this it follows that 



QocR 2 . 

 To anticipate the effect of a variation of D, we may simply 

 extend this last statement, in the form 



V 2 

 QocR 2 exp- 

 and what remains to be proved is that Q varies inversely as D 2 . 

 Generally, we may connect the experimentally measured 

 quantities D, V, Q by the equation 



y2 



Q = E7D 2; 



and then, to complete the proof of the Law of Squares as 

 already stated in (1), we must find that the quantity E, or the 

 equivalent quantity 



QTD^' 



preserves a constant value when D varies as well as V. 



19. Method of Experiment. — It was necessary here to obtain 

 a set of exactly measured values of D. The following method 

 was adopted, as well within reach and at the same time suffi- 

 ciently accurate. I assume that the reader remembers what 

 has been said about the upper conductor and its adjuncts, the 

 ball-headed screws, rafters of glass, and lateral supports of 

 thick plate (2). A sheet of thin plate glass, chosen for its 

 apparently perfect uniformity, was cut up into a number of 

 equal slips, of about the same length and breadth as the upper 

 conductor. The joint thickness of sixteen of the slips was 

 exactly one inch. When the slips were laid side by side on a 

 plane table, and tested optically and otherwise, they were found 

 to differ sensibly, though very little, in thickness. Four of 

 them were selected that showed no such difference ; and these 

 were reserved as standard slips, the thickness of each of them 

 being taken as unit value of D. 



The pieces being all in final position for experiment, the 

 cell empty and open, and the upper conductor screwed close 

 up to the rafters, the distance between the two conductors 

 was found to be rather greater than 1. A standard slip was 

 laid on the floor of the cell, between the two conductors ; and 



