Measurements with Quadrant Electromete 



239 



[N.S.] 



show that in accordance with the theory first given by Sir J. J. 

 Thomson, 1 andsince developed by Stewart,* Beattie, 8 and others, 

 the effective capacity of the quadrants, as determined by the 

 method of mixture with a known capacity, varies as the square 

 of the potential given to the needle. The figures in the third 

 column of this Table have been calculated from the equation 

 C = 245 + 0-0114 V 2 . In order to make the initial deflection of 

 the electrometer approximately constant, the potential applied 

 to the quadrants was varied, but in no case exceeded 10ls 

 volt. 



Table VII. 



Needle charged to 



2-0 volts 



5-8 

 15-6 

 197 



23-8 



26-0 

 29-6 

 34 8 



37-4 



414 

 46-5 

 49-4 



Capacity observed 



Capacity calculated 



24-6 E.S.U 



24 <> 



27 3 



28*9 



30-0 



32-2 



34 5 



38 3 



40-5 



44-0 



49-2 



52 5 



Allowing for the capacity of the key, that of the quadrant 

 system with an uncharged needle was therefore about 16 E.S.U. 

 (c) Observations of the insulation resistances of the arrange- 

 ments described in this paper, made from November 11th to 

 18th, 1913, gave the following results : 



Quadrants + key 

 Ebonite of condenser 



Sulphur plug 



0-4 to 0*6 x 10* megohms. 

 2-0 to 6-0 x 10 » 

 0-2 to 0-6 x 10 » 



7 moisture, its insulation 



Though ebonite is much affected by moisture, its 

 resistance is extremely high in a thoroughly dry atmosphere. 

 The arrangements described in section 2 of this paper 

 cannot be regarded as entirely satisfactory, as moist air is 

 still allowed free access to the inside of the electrometer case, 

 through the four small holes left for the passage of the threads 

 to the key. Temperature differences amounting to four or five 

 degrees are set up when the laboratory doors are kept open. 

 As the capacity of the case is about forty litres, a difference of 

 five degrees at a mean temperature of 30° C would set up a flow 



1 Phil. Mag. 



a Phys. Revi 

 3 Electrician 



1898. 

 XXI, 4; Oct. 1905; p. 22'! 



; Aug- 12, 1910 ; p. 729. 



