1898.] Electrification by Ultra-violet Light. 411 



.". integrating both sides we obtain 



a E « T 

 d = -T . — . u, 



a it 



The distance d is determined experimentally, and E and T are 

 constants of the transformer circuit, so that u is known. Since 

 the electrometer circuit is insulated before the e.m.f. is applied to 

 the lower plate, the true potential difference between the top and 

 base plate is less than the potential of the lower plate on account 

 of the induction effect between the two plates. In consequence 

 of the rapidity of the alternations the electrometer needle shows 

 no movement due to the induction effect, so that it is necessary 

 to determine the correction to be applied when the needle of the 

 electrometer remains at rest. This may be simply done as 

 follows : 



The two quadrants of the electrometer are insulated from each 

 other, and then a steady E.M.F. is applied to the base plate. The 

 electrometer needle is deflected, but is brought back to its original 

 position by applying a suitable E.M.F. to the other pair of quad- 

 rants. Let r be the ratio of the applied potential to the potential 

 V of the base plate. The potential of the electrometer circuit is 

 rV and the true difference of potential between the plates is 



(1 - r) V. 



The correction is thus made by putting (1 — r) E instead of 

 E in the equation that determines the velocity. 



The following is an example of a determination of the velocity 

 of an air ion travelling through air, 



d = '8 cm., 



T=^sec. 



Reading of alternating voltmeter 95, the maximum value of 

 the e.m.f. E is therefore given by 



# = 95V2, 



The correction for induction between the plates was 5 per cent., 

 so that the final value is 



u = 144 cm. per sec. 



A large number of determinations have been made for the 

 velocity of the air ion, and the mean of the results gives a velocity 

 of about 



T45 cm. per sec. 



