Velocity of ft- particles in passing through Matter. 627 



moment : the needle remains at rest. In order to put the 

 rotor in action we must give it an impulse, and the starting- 

 rotation can be brought up to the value for synchronism, the 

 rotor being, as we say in French, " accroche" by a physical 

 process I established in the paper already referred to. 



Now, we shall obtain the best conditions for having a 

 suitable impulse by bringing first the rotor into resonance 

 with the help of the auxiliary field. Cutting off at that 

 given moment the direct current, we observe that the needle 

 turns round, and if we illuminate it with a lamp put on the 

 main alternating current, w T e can observe in the dark that its 

 shadow is at rest, and verify thus by the ordinary strobo- 

 scope method that the rotor is turning at the synchronous 

 speed, and is " accroche." We can now remove if we like 

 the electromagnet A and the rod D. If we move a magnet 

 near the needle we can bring it again to rest, and thus verity 

 another law of the synchronous motor: it falls out of step 

 when the resistant moment becomes too large. 



G. It' we desire to show the resonance experiment roughly 

 — for example during a lecture — the most expeditious way is 

 to raise the current I to such a value that the needle is quite 

 motionless for a given position of the iron rod. We then 

 pull the iron rod back with a slow motion. At a given 

 moment we shall observe a maximum deflexion of the needle, 

 and if we continue to move the iron rod, we observe that the 

 vibration of the needle decreases in amplitude, and the needle 

 comes again to rest. 



The experiment can be performed by making use of 

 needles of different inertia and appropriate frequencies of 

 the alternating current. 



Oxford, Prof. Townsend's Laboratory. 

 July 1915. 



LXIII. The Decrease in Velocity of fi-par tides in passing 

 through Matter. By W. F. Uawlinson, M.Sc* 



rr\HE decrease in velocity of /^-particles in traversing 

 Ml. matter is difficult to measure on account of the marked 

 scattering which they suffer. W. Wilson | examined the 

 problem in detail by an ionization method ; but as his initial 

 beam was not homogeneous, his final results prove only that 

 there is a reduction in the average velocity of /3-rays in 



* Communicated by Sir E. Rutherford, F.R.S, 

 t Proc. Roy. hoc. A. lxxxiv. p. 141 (1910). 



2 S2 



