Electrometer as a Wattmeter and Voltmeter. 



357 



recent construction, and differs from the other principally in the 

 omission of the guard tube in the immediate vicinity of the needle 

 surrounding a portion of the needle axis, and the wire connecting 

 the needle to the acid inside the jar. The induction plate employed 

 in the old form is done away with, and the terminals are perma- 

 nently fixed to the quadrants in this new instrument, otherwise, so 

 far as the author can see, they are identical. These instruments 

 belong to Dr. J. Hopkinson, F.R.S., the old form being the same 

 that he has used for many years, and in connexion with which he 

 read a paper before the Physical Society on March 14, 1885.* 



Verification of Clerk MaxivelVs Formula. 



In the paper just alluded to, it is shown that the sensibility of the 

 instrument (No. 71) increased with the charge on the needle up to a 

 certain point, and that for further increase of the charge on the 

 needle the sensibility diminished. The complete explanation of this 

 is not given, and the author believes Professors Ayrton and Perry 

 were the first to point out that this effect is due to the portion of the 

 guard tube in the immediate neighbourhood of the needle. 



To test this point in the new instrument a Kelvin vertical electro- 

 static voltmeter was placed across the needle and case, and a con- 

 stant electromotive force applied to the quadrants, one pair being put 

 to the case. The jar was then charged by sparks from an electro- 

 phorus, and readings taken on the voltmeter and electrometer 

 scale. The charge was continually increased until disruption 

 occurred between the needle and the lantern which supports the 

 idiostatic gauge. Up to about 2,450 volts on the needle the sensi- 

 bility increased, and so far as the author could see the needle was 

 farther deflected as the charge was increased up to the point of dis- 

 ruption, the spot of light being then off the scale. No great care 

 was taken with this experiment, since it was only carried out for the 

 purpose of ascertaining if diminished sensibility could be obtained 

 with further increased charge. The results are given in fig. 1. In 

 Clerk Maxwell's ' Electricity and Magnetism,' vol. 1, p. 273, edition 

 1873, it is shown that the deflection of the needle of a quadrant 



electrometer should vary as (A— B)^C — ~~^~^> where C is the 



potential of the needle, and A and B the potentials of the two pairs 

 of quadrants. In fig. 1 the E.M.P. between the quadrants was less 

 than 1 volt, and was constant. By the formula the quotient C/0 

 should in this case be constant where is the observed deflection. 

 It varies in arbitrary units from 0*55 to 0*11 as the value of varies 

 from about 550 to 2,450 volts. This is working the instrument far 



# See ' Philosophical Magazine,' April, 1885. 



