940 Dr. J. S. G. Thomas on Mutual Action of Convection 



inverse square of the distance between the centres of the two 

 nuclei. 



(2) The variation in the number of H particles projected 

 within a given angle with the velocity of the a-rays has been 

 observed over a wide range. It is shown that for a-rays of 

 high velocity the variation is in the opposite direction to 

 that given by the inverse square law; for a-rays of range 

 less than 2 cm., velocity less than 1*26 x 10 9 cm. per sec, 

 however, the collision relation is about the same as that 

 given by the inverse square law. 



(3) The experimental collision relation is compared with 

 those calculated by Darwin for various models of a particle, 

 and the conclusion is reached that the a particle behaves in 

 these collisions as an elastic oblate spheroid of semi-axes 

 about 8 x 10~ 13 and 4 x 10 -13 cm., moving in the direction 

 of its minor axis. Outside this surface the force varies 

 approximately as the inverse square of the distance from 

 the centre of the spheroid. 



In conclusion, we desire to express our best thanks to 

 Sir Ernest Rutherford for his interest and advice throughout 

 the course of the experiments. 



CI. The Mutual Action of the Convection Currents arising 

 from two fine heated Platinum Wires. By J. S. G. 

 Thomas, D.Sc. (Loud.), B.Sc. (Wales), A.B.C.S., A.I.C.< 

 Senior Physicist, South Metropolitan Gas Company, 

 London* . 



Introduction. 



A TYPE of hot-wire inclinometer wherein two fine heated 

 XJL platinum wires, parallel to one another, were subjected 

 to the mutual thermal effects due to their respective free 

 convection currents, such thermal effects being dependent 

 upon the inclination to the horizontal of the plane of the 

 wires, was recently described by the author f . The effects 

 to which attention was directed in that paper are of some 

 consequence in a variety of directions, e. g. in con- 

 nexion with the theory of the hot-wire microphone as 

 described by Tucker and Paris |, Likewise in the design 



* Communicated by the Author. 



t Proc. Phys. Soc. vol. xxxii. Part v. pp. 291-314 (1920). 



t Phil. Trans. Poy. Soc. ; A. 592, vol. 221, pp. 389-430 (1921). 



