Electrometer as a Wattmeter and Voltmeter. 361 



from 0*001 to 1 microfarad, its capacity being 1 microfarad during 

 the experiments, the results of which are given in Table II. 



Before giving the results of the experiments it is well to explain 

 the method adopted of treating the curves for the purpose of arriving 

 at the average Watts due to the alternate current, the relation be- 

 tween which and the deflection of the electrometer used as a Watt- 

 meter it is desired to find. It is also necessary to examine the limits 

 of accuracy obtainable by this method. In any one experiment the 

 frequency employed is kept constant as nearly as possible : the phase 

 difference between current and potential is adjusted to any desired 

 value and the amplitude of these quantities is kept constant by ob- 

 serving their square root of mean square values on the instruments 

 D and F. The revolving contact maker M is then set to different 

 positions of the phase, the number employed being at least ten equal 

 divisions to the half period, and for each position, readings taken on 

 the electrometer Q 2 when the switch Si is in each of its two positions. 

 If the deflections so obtained be plotted in terms of the position of 

 the revolving contact maker M, the forms of the two curves are 

 those due to the instantaneous values of the potential difference 

 applied to the needle of the electrometer Q 1? and the current which 

 gives the form of potential difference applied to the quadrants of Q lt 

 By multiplying each of these deflections together, and by a suitable 

 constant involving the square of the sensibility of Q 2 and the resist- 

 ances To ' f T3, or 7*3 ; r 5 , r G , the instantaneous rate at which work is 

 being done by the alternate current can be inferred in Watts. The 

 average of these over a half period gives the average rate, and this 

 can be obtained by plotting the instantaneous product and taking 

 the area with a planimeter, or the average of the algebraic sum 

 during a half period can be taken. The author found the latter 

 method agreed so well with the former when the number of inter- 

 vals at which observations are taken is ten, that he has adopted it 

 in this paper, that is to say, the two electrometer deflections for a 

 given position of M are multiplied together, the average of these 

 taken over half a period, and such average multiplied by a constant 

 to reduce to Watts. 



The best way, perhaps, to test the limits of accuracy is to adjust 

 current and potential until they are exactly in phase. The volt- 

 meter F and amperemeter D give the square root of mean square 

 values, and the product of these should agree with the average 

 results obtained from the curves. The time required to take one 

 set of observations is generally about twenty minutes, during this 

 time an average for volts, amperes, and frequency is taken. The 

 author finds from experience that if care be taken an agreement 

 between the results got from the curves, and from the product of volts 

 and amperes, can be obtained to within one or two per cent. It 



