ox A CHARGED El.ECTRlC CONDENSER MOVING THROUGH SRACE. 
177 
value was confirmed by compaiisou with another known capacity by De Sauty’s 
method. 
Measurement of the ControUuuj Couple. 
A piece of laass rod, of length 3‘35 centims., and diameter '07 centim., and 
weight 20'6 grams, was attached by its centre to tlie suspending strip with its long- 
axis horizontal. The time of oscillation of this was observed, and gave the value 
2 mins. 17'8 secs. The value of the couple per unit twist deduced troin the above 
data is equal to •042(1 (J.G.S. 
Calculation of N ( . 
C'apacity of condenser ‘OUbf m.f. 
Couple })er unit twist of wire •0426 (f.G.S. 
Potential difierence = 20(-)O volts = - 3 " electrostatic units. 
Electrostatic energy = T 
The distance of tlie sun from the earth Is I'oO X 10 ^® centims., so that 
277 X 1'50 X 10'-'^ centim. centim. 
u = - =3X10" 
3()5 X 24 X 60 X bU sec. sec. 
Taking v (velocity of pro])agatIon) as 3 X I (T^, we have {ujv)~ = 10“'^. 
The couple due to tlie drift wlien the condenser is in its best position is N . 
Let d he scale defiection at distance of 1 metre; then angular detlection is f 
Equating the balancing couples. 
•043 X 
d 
200 
lo-n 
/ •0037 X v~ 
V 10^-^ 
so that d is 3^4 centims. That is to say, neglecting the jiroper motion of the sun, 
and under the best conditions, when charged to a potential of 2000 volts the 
defiection of the condenser should amount to 3^4 centims. 
Observations. 
In order to he sure that the condenser was not held in any jmsitlon. It was set 
oscillating uncharged and readings were taken every quarter of a minute ; giving 
the curve (fig. 10). 
VOL. coil.—A. 4 A 
