PHYSICS 193 



rotating magnet to rest ; the electron returns to a distance 

 with its old velocity and energy, having experienced what is 

 practically a perfectly elastic encounter with the atom. But 

 if the energy of the electron exceeds a certain critical amount, 

 it is able to pass right past the magnet (that is through the atom) 

 and travel to a great distance on the other side with its energy 

 reduced by this critical amount, which is now retained by the 

 rotating magnet. This is a very striking parallel to the now 

 well-known phenomenon of " single-line " spectra first dis- 

 covered by Franck and Hertz in 19 14, and since amply confirmed 

 by the researches of McClennan, Horton, Goucher and others. 

 In this an atom is caused to emit radiation of frequency v by 

 coUision with an electron, the amount of kinetic energy of the 

 electron which is absorbed by the atom being h v, (h = Planck's 

 constant) : an electron whose kinetic energy before the en- 

 counter is less than h v is incapable of stimulating the atom to 

 emit the radiation and is repelled from the atom without any 

 loss of energy. The manner in which the atom can pick up 

 just a definite amount of energy, neither more nor less, has been 

 suggested above, and Whittaker's equations lead to a value 

 2e^m^/l for the critical energy, where e is the electronic charge, 

 m the pole strength, and I the moment of inertia of the magnet. 

 The mathematics can be applied in a similar manner to any 

 magnetic system which, like a Weber element, can be set in 

 rotation by a magnetic field. The angular velocity retained 

 by the magnet is given by 



U = e m 0) 



where U is the critical energy. 



It constitutes a strong element of support for this explana- 

 tion of quantum absorption of energy that it postulates no 

 structure in the atom beyond one of a kind which has already 

 been introduced to account for a totally different class of 

 phenomena, those of induced magnetisation. 



It remains to be seen how the energy so absorbed in these 

 discrete quantities and resident in the atom as energy of a 

 magnetic current, becomes transformed into the radiant form. 

 At this point Whittaker frankly forsakes the model and simply 

 postulates that a magnetic current can be replaced by an 

 " electric shell," i.e. a condenser, just as we look upon a mag- 

 netic shell and an electric current as equivalent. Such a con- 

 denser discharging is likewise equivalent to the famihar Hert- 

 zian oscillator of the text-books whose frequency is given by 



V = i/27r\/LC 



where L is an inductance and C a capacity. We can assume that 

 the vibrators in different atoms merely differ in scale, but are 



