534 Dr. W. F. G. Swann on the Pulse 



and the flow after 10" 3 sec. is only "077 X 10~ 6 , instead of 

 0*87 x 10~ 6 . Thus if the galvanometer is one which gives a 

 throw of 200 mm. per micro-coulomb, with a period of 

 15 sees., we may say that after so short a time as y^oo °^ 

 the period all unsteadiness of the currents has sensibly 

 vanished. The conclusion seems to be that far too much 

 has been made of the importance of a continuous balance, 

 and that unsteadiness of the needle must be otherwise ex- 

 plained. In cases where an ordinary condenser is employed, 

 an ample explanation is provided by the phenomena often 

 referred to as " residual charge.'"' In experiments where an 

 air-condenser (or no condenser at all) is used, there is still 

 something analogous to the ordinary condenser. It is certain 

 that the capacity of the connecting wires is not always as 

 thoroughly negligible as is usually assumed, especially when 

 twisted leads are employed; and it seems not impossible that 

 these may in some instances play a similar part in preventing 

 the operator from securing a satisfactory balance. 



LVII. The Pulse Theory ofX Rays, y Rays, and Photoelectric 

 Rays, and the Asymmetric Emission of ft Rays. By W. 

 F. Gr. Swann - , D.Sc, A.R.C.S., Assistant Lecturer in 

 Physics at the University of Sheffield *. 



IT is well known that when a beam of X rays or ultra- 

 violet rays falls upon a thin sheet of metal, the values 

 for the number and velocity of the ft rays produced are 

 greater for the emergence ft rays than for the incidence 

 ones The explanation of this fact on the wave theory is 

 generally considered to be attended by difficulties so serious 

 as to necessitate the abandonment of the pulse theory in 

 favour of a corpuscular hypothesis. Practically the same 

 arguments which support the corpuscular theory for X rays 

 also support it for ultra-violet light, so that if we abandon 

 the wave theory for one, we are almost forced to abandon it 

 for the other. In the following paper I have endeavoured 

 to show to what extent the want of symmetry in the expul- 

 sion of ft rays may be reconciled with the pulse theory, and 

 also to discuss certain other points concerned with the pro- 

 perties of X rays, &c. 



There are two views which we may take as to the origin 

 of the ft rays which an electromagnetic train releases. Tliey 

 may come from those free electrons which exist between the 

 molecules of the metal, or they may be torn out of the atoms. 



* Communicated by the Author. 



