70 Prof. J. A. Fleming on the 



in which the " throw " of the same galvanometer is observed 

 when the same condenser is charged by contact with two 

 points concerning which we require to know the potential- 

 difference, gives us the means of calculating the electrostatic 

 potential-difference in volts. A condenser of "987 microfarad 

 capacity carefully determined was charged to a potential of 

 54 volts and discharged through a certain ballistic galvano- 

 meter having a needle whose periodic time of vibration was 

 about three seconds. The resulting " throw " of the galvano- 

 meter was 5° 30'. Hence a discharge of 54 x •987 = 53*3 

 microcoulombs through the galvanometer produces a " throw" 

 of 5° 30'. Neglecting a very small correction for the loga- 

 rithmic decrement, in this case not of importance, we have 

 for the ballistic constant R the value 



53'3 = R sin J(5° 30') = R x -04798 ; 



hence R=1110. 



The same condenser was then connected between the 

 middle plate and positive electrode of lamp No. 4 and then 

 discharged through the same ballistic galvanometer. The 

 lamp was subjected to a working pressure of 39 volts as de- 

 termined by a corrected voltmeter attached to the electrodes 

 of the lamp. The charge of the condenser was sent through 

 the ballistic galvanometer, and a " throw " of 4° obtained. 

 If v is the potential-difference between the middle plate and 

 positive electrode of the lamp, we have the following equation 

 for v in terms of the ballistic constant and angle of " throw": 



•987^ = 1110sin2° 



= 1110 x -0349 



or 111x349 __ . 



v = q^= = 39 nearly. 



The potential-difference between the middle plate and the 

 positive electrode as determined by this method is therefore 

 exactly the same as the potential-difference between the positive 

 and negative electrodes of the lamp. In other words, when the 

 filament is brought to full incandescence, the middle metal 

 plate is brought to the same potential as the negative elec- 

 trode of the lamp. This observation was repeated with several 

 other lamps having middle plates in various positions and of 

 various forms, and always with the same result, viz., that the 

 potential of the middle plate when insulated is brought down 

 nearly to that of the negative electrode. 



