CONTEMPORARY ADVANCES IN PHYSICS 199 



reader may consult Fig. 4, in order to notice that one of the three 

 showers there depicted sprang from a place in the plate to which no 

 charged particle came. This suggests that a photon may cause a 

 shower, and that the multiplication of a shower already begun is due 

 to the action of its charged particles and of its photons both. 



Two classes of charged particles begin to take shape : the penetrating 

 ones on the one hand, the shower particles and the shower-producing 

 particles classified together on the other. To bring out another 

 aspect of the distinction, I now turn to the data underlying Fig. 8. 



These data are derived from cloud-chamber photographs such as 

 Fig. 9 exemplifies. If the track of a charged particle is sensibly curved 

 in such a magnetic field as it is possible to apply to a Wilson chamber, 

 it may be possible to infer the momentum and the energy of the 

 particle.^ I digress to give the formulae, so as to make it clear just 

 what can be deduced from what amount of knowledge. The ele- 

 mentary procedure consists in pointing out that the charged body 

 describes a circle in the plane perpendicular to the magnetic field, 

 and that consequently the force exerted on it by the field is to be 

 equated to the product of its mass by its centrifugal acceleration. 

 Putting ne for the charge (in electrostatic units) of the corpuscle, 

 m for its mass, v for its speed and p for the magnitude of its momentum 

 in the plane normal to the field, p for the radius of the circle and H for 

 the field-strength, and writing down the two members of the equation, 

 one finds: 



Hnev/c = mv^jp, (1) 



p = ine/c)Hp. (2) 



These equations remain valid when (as usually is the case with cosmic- 

 ray electrons) the speed is so great that relativistic mechanics must be 

 used instead of ordinary. At such high speeds equation (2) retains 

 its aspect. Equation (1) may also be left unaltered, but one must be 

 sure to remember that m is a certain function of v: 



m 



= WoV/1 - vyc\ (3) 



Wo being known as the "rest-mass" of the body. 



* Curvatures of tracks being so very important in this field of research, it is 

 necessary to examine with the greatest of care into all of the causes (apart from 

 magnetic field) which may produce or afTect them. Notable among these are 

 currents in the gas, which are especially obnoxious if there is a metal plate in the 

 chamber. Indeed it seems strange that the currents should not be more hampering 

 than they are, considering the expansions which occur. Sometimes people observe 

 that in the absence of magnetic field, there is a slight curvature of the tracks; then 

 in the presence of magnetic field, they deduct this amount from the curvatures 

 observed. The papers of Anderson and Blackett abound in information on these 

 delicate questions. 



