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Proceedings of the Poyal Society of Edinburgh. [Sess. 
Equations (20) and (21) show that the two variables \J/- and g oscillate, 
the energy of the system existing alternately as a magnetic current in 
the magnetic wheel and an electric current in the wire : the frequency of 
oscillation is found at once from (20) and (21) to be 
M 
7T \/ 
/aa 
( 22 ) 
The quantity ^ now has the same dimensions in every system 
of units ; and by reasoning similar to that in § 4 we obtain the equation 
A 
A 27t 
(23) 
Eliminating A and A between equations (8), (22), and (23), we have 
U = hv , 
as was found in equation (19) above. 
§ 6. The Explanation of Photo-electric Phenomena. 
In the discussion on Alay 8, I gave the following indication of the way 
in which photo-electric phenomena would be interpreted on the basis of 
the above theory of quanta. 
Suppose that light of frequency v is incident on a metal. There are 
grounds for supposing ^ that in a complex body such as a metal there are 
systems whose natural frequencies of vibration are distributed almost 
continuously over a wide range of frequencies. We shall therefore 
suppose that the incident light provokes resonance in a system within the 
metal, consisting of a magnetic wheel linked with a wire circuit, as 
described in § 5 above; so that the system absorbs energy from the 
incident liofht and the magfnetic wheel is thus set in rotation. This 
absorption of energy from the incident light proceeds continuously. 
Suppose now that one of the free electrons of the metal happens to be in 
the vicinity of the magnetic wheel : the field due to the rotation of the 
wheel will cause the free electron to be sucked inwards to the wheel, as 
follows at once from the equations of § 3. If the energy of rotation of the 
magnetic wheel is less than the amount \J = hv of §§ 3-5, the free electron, 
after having been sucked in a certain distance, will be rejected without 
passing through the wheel or having (in the long run) abstracted any 
energy from it ; but if the energy which the magnetic wheel has absorbed 
* Cf . J. J. Thomson, Phil . Mag ., 37 (1919), p. 434. 
