362 



Mr. E. Wilson. The Kelvin Quadrant 



must be remembered that for each position of the contact maker, 

 four observations on the electrometer (Q 2 ) scale have to be obtained; 

 that is, two for potential and two for current corresponding to the 

 two positions of S 2 for each position of Si, the difference in each 

 case giviug the net double deflection. This method is best, as it 

 eliminates any zero error there may be. In working the electrometer 

 Q 2 , a wooden tapper or mallet is employed, since in every electro- 

 meter there must be viscosity due to the fluid, and by gently tapping 

 the slate base for each deflection very consistent results can be ob- 

 tained. This viscosity is greater in winter, and it is advisable to 

 keep the instrument in a warm room, although with this method of 

 tapping the author does not find this necessary. The greatest 

 trouble in the use of the electrometer undoubtedly arises from dust 

 settling on the surface of the acid in the jar, thereby making the 

 angular movement of the wire hanging from the needle smaller than 

 it would be if such brake action did not exist. This takes place 

 when the acid in the jar is old, and if the surface be agitated by 

 blowing through a glass tube near where the wire dips into the acid 

 it can be to a great extent remedied. Whatever the state of the 

 acid the author finds he gets the most consistent results by gentle 

 tapping. The electrometer Q t is not so sensitive as the old form Q 2 , 

 and the effect due to the acid in it has not given so much trouble. 

 The sensibility of Q 2 when the idiostatic gauge is adjusted is such 

 that one Clark cell gives a deflection from zero of 10J inches on a 

 scale 12 feet from the mirror. The potential of the needle is in this 

 case about 350 volts. 



Experimental Results. 



In making a thorough test of the electrometer as an alternate 

 current Wattmeter we have the following variables to deal with : — 



1. The frequency of the alternate current. 



2. The phase difference between current and potential, that is 



between C and A or B. 



3. The amplitude of C and A or B. 



4. The shape or wave form of the curve of potential and current. 



The results obtained are tabulated in Table II and are divided into 

 three groups (a) (b) (c). In group (a) two frequencies are given, 

 namely 41'6 and 83 complete periods per second. The potential on 

 the needle is constant at about 100 volts (-v/mean 2 ). The phase 

 difference between potential and current and the current itself are 

 each varied. When the phase difference is zero, it is only necessary 

 to take the product of the ^/meair values to deduce the Watts, 

 although in section (b) three instances are given in which for 

 phase difference zero, the Watts are deduced by both methods. The 



