662 Mr. R. Hargreaves on tlie Difference between 



made, comparing the intensity of the radiation when the 

 emanation was subject to small and to large accelerations. 

 The results of these tests were as follows : — 



1. An increase due to acceleration of (*08zt'07) per cent. 



2. A decrease due to acceleration of (\17 + *09) per cent. 



3. A decrease due to acceleration of ('lT + 'O?) per cent. 



4. An increase due to acceleration of (*02±'10) per cent. 



The average of all these tests indicates a decrease in the 

 intensity of the gamma radiation of (06±'04) per cent. It 

 is probable, therefore, that an acceleration of 20,000 times 

 gravity produces no effect on the intensity of gamma radiation 

 as large as one part in a thousand. 



In order to explain the small degree of radioactivity of 

 the earth's interior it would be necessary to assume a 

 comparatively large change due to an increase in the gravi- 

 tational acceleration. The negative result of this experiment 

 therefore shows that vve must look elsewhere for the cause 

 of the confinement of the earth's radioactivity to its surface 

 crust. 



I wish to thank Professor Rutherford for proposing thi$ 

 problem to me, and for his helpful suggestions and encourage- 

 ment as the work progressed. 



Cavendish Laboratory, 



Cambridge University. 

 Feb. 3, 1920. 



LXVI. The Difference between Magnetic and Electric Energies- 

 as a Pressure. By R. Hargreaves, M.A.* 



IN any electromagnetic field the pressure on a perfectly 

 reflecting surface is normal, and is measured by the 

 difference between the magnetic and electric energies (per 

 unit volume) at the surface. This theorem, the subject 

 of the present paper, may be viewed in conjunction with 

 an analogous theorem in Hydrodynamics which appeared 

 in this Magazine |- There it was shown that Kelvin's 

 kinetic potential for the motion of solids in infinite liquid, 

 with circulation taken into account, could be got by taking 

 a volume integral of the pressure. The analogy is at once 

 apparent, and may be helpful in explaining the raison d'etre 

 of I he kinetic potential in Electromagnetics. For though 



* Communicated by the Author. 



t "A Pressure-integral as Kinetic Potential/' Sept. 1908. 



