1^22 J/r Ca))ipbc!L Disconfijiiiltic.'^ in Liaht Eiuis.-^'io)!. 



§ \4f. The rosistnnoo bv which the oloctronuMor was shuntod 

 Nvns of the order of j\i K.s. U. (^}) x 10'" ohinsV Sueh resistanees 

 ;vre too low to be provided bv ionised air in the manner now 

 associated with the name of Bronson : niovco\ or, such air re- 

 sistances give rise to fluctuations, due to the tinitude of the 

 number of rays bv which the air is ionised, which wonUl be of 

 the same order of magnitude as those thai are h(Me invest.igatcii 

 Several substances were ti'ied as materials tor the required re- 

 sistance, inchidino; graphite, copper oxide, Phillips' conducting 

 glass and vaivious liquids. The latter e\ entualiy proved the most 

 satisfactory. A mixture of Xylol ami b'thyl Alcohol (frou\ JH) 

 to 5 parts of the former to 1 of the latter) was ultin\ately em- 

 ployed in a tube oi' such a form that the distance between the 

 platinum electrodes could be varied. One end o( the resistance 

 was connected to the common electrode of the cells, the other to 

 a potentiometer arrangement, the need for which will be explained 

 later. 



The insulation of the electrode system, of sulphur and amber, 

 was such that its conductivity was certainly less than O'Ol of that 

 of the artificial resistance: its conductivity is therefore neglected. 



The value of R was determined directly by Ohm's Law. One 

 of the cells being kept dark, the light falling on the other was 

 adjusted to such an intensity as to give a convenient current 

 through the cell. The magnitude of this current was measured 

 by the electroscope and capacity box — the electrometer being 

 taken out of the circuit. The electrometer was then connected 

 again and such a potential applied to the potenticMneter /*, that 

 the electrometer showed no detlection. Then the potential between 

 the ends of the resistance \vas such that the current through the 

 resistance was equal to that through the photo-electric cell. In 

 this manner it could be shown that the resistances used obeyed 

 Ohm's Law with an accurai'v of 1 '7o *-"*^'*^^i' •"'' ^*!>i\i^«^ ^">f I'-i^- ^'"^>'" 

 1'5 to 0002 E.s.U. The polarisation E.M.F. of the liquid resistance 

 was certainly less than lO""* E.S.U. 



§ 15. The only quantity in equation (11) the measurement of 

 which presented any dithculty was the capacity. T'he capacity 

 of the electrometer was a considerable part of that of the whole 

 electrode system, and it is well known that, this capacity is not 

 constant, but varies with the delleciit>n and, so far as l can n\ake 

 out, with the rate of movement, oi' the needle. The most hopeful 

 way of obtaining an etll'ective value for the capacity of the electroilc 

 system appeared to be by measuring the value of 11 and of 

 /) = l/BG : p was determined by observing the rate of decay of 

 the detlection of the electrometer, when the electrode system was 

 raised to a given potential and then insulated. (The initial part 

 of the decay was neglected, because of the inlluence of the time 



