Mr Campbell, Discontinuities in Light Emission, 323 



constants of the electrometer.) If the capacity of the system 

 varies with the deflection, the resulting curve of deflection against 

 time ought not to be exponential. As a matter of fact, it ap- 

 peared to be perfectly exponential, but, on. the other hand, the 

 calculated value of the capacity increases regularly with a decrease 

 in R. On decreasing R from 0*13 to 0'013 G appeared to increase 

 from 380 to 600. 



That this peculiarity was due to the electrometer was proved 

 by the fact that, if the electrometer were disconnected and the 

 decay observed by the electroscope, the capacity being varied by 

 means of the capacity box, perfectly consistent results were 

 obtained. A great deal of time was wasted in an inquiry into 

 this matter and no conclusive information was obtained. The 

 matter is not of first class importance, because in comparing 

 fluctuations, care would, of course, be taken to keep the instru- 

 mental constants the same : and, in estimating the absolute value 

 of N{(o^ + ri^), the errors in the measurement of the fluctuations 

 arc likely to be as serious as the uipicertainty in the value of 

 the capacity. 



§ 16. It will be well to consider here for a moment whether 

 the magnitude of the fluctuations to be expected on the theory 

 of light which is under discussion is such that it is likely to be 

 capable of detection and measurement. On Planck's theory some 

 estimate of this magnitude can be made. 



According to that theory the quantity of energy (e) contained 

 in one light disturbance is given by 



e = 6*5 X 10~^^ V, where v is the frequency of the light. 



Hence if we know the amount of energy in each of the electrons 

 shot off from the photo-electric substance, we can deduce the 

 average number of electrons shot off by each light disturbance : 

 i.e. the quantity to. The application in this manner of Planck's 

 theory to the photo-electric effect has been made by Einstein* 

 and by Joff^f, the latter giving numerical calculations based upon 

 the work of Ladenburgj'. It appears from these calculations that 

 the energy given by the light to the electrons shot out from zinc 

 is about one-third of that contained in each light disturbance : so 

 that Q) is about 3. For the alloy of sodium and potassium, which 

 is more photo-electric, co is certainly greater. But, since a mini- 

 mum estimate is desired, we will take eo as 1, and will put 

 v" = — though, if CO is as small as 1 it is probable that tj'^ is of 

 the same order as co. 



* Einstein, Ann. d. Phys. xx. 1906, p. 199. 

 t Joff6, Ann. d. Phys. xxiv. 1907, p. 939. 

 X Ladenbnrg, Pliys. Zeit. viii. 1907, p. 590. 



