2 racks of a Particles from Radium A in Sensitive Films. 129 

 Summary and Conclusion. 

 The results obtained ma}' be summarized as follows : — 



(1) No variation in the dielectric constant could be de- 



tected up to gradients of 3 x 10 6 volts per cm. 



(2) The conduction current increases very rapidly with 



the gradient when the latter is large. The formula 

 log (J = A 4- log X + BX represents the current within 

 the limits of error over a wide range. A possible 

 explanation of this formula is suggested on the basis 

 of r distribution of electronic velocities in accordance 

 with Maxwell's law. 



In conclusion I wish to express my gratitude to Mr. H. 

 Thrift, F.T.C.D., for the assistance he has so kindly given 

 me in reading the galvanometers. 



Physical Laboratory, 



Trinity College, Dublin. 

 April 24, 1916. 





I 



IX. The Tracks of the a Particles from Radium A in Sensi- 

 tive Photographic Films. By H. Ikeuti, Research Student 

 Imperial University, Tokyo *. 



[Plate IV.] 



WHEN a point source of a. rays is placed on a photo- 

 graphic plate, the paths of radial a rays can be traced 

 in the film. If the rays are homogeneous, all the tracks will 

 terminate on a spherical surface, whose radius is equal to 

 their range in the substance. The consequence is that the 

 radial tracks appear as a halo. Haloes obtained in this way 

 with radium C have already been illustrated in a previous 

 paper by Mr. Kinoshita and myself f . Although no parti- 

 cular difficulties were encountered in obtaining them with 

 radium C, w T e were not able to get them with radium A. 



Since the above paper was published, I made further 

 experiments and obtained lately some photographs on which 

 the a-ray tracks of radium A are clearly visible. The 

 method employed consisted simply in knocking a plate at 

 the film side with a small iron ball, which had just previously 

 been exposed to radium emanation for a few minutes. The 

 plate was developed usually half an hour later, and examined 

 under a microscope on drying. In the present experiments, 

 Ilford Process Plates only were used. 



* Communicated by Prof. H. Nagaoka. 



t S. Kinoshita & H. Ikeuti, Phil. Mag. March 1915. 



Phil. Mag. S. 6. Vol. 32. No. 187. July 1916. K 



