238 



Prof. J. Zeleny. The Velocity of 



DESCEIPTION OF PLATE. 



A. Solar spectrum, purposely out of focus. 



B. a Aquilae. 



C. iS Arietis, purposely out of focus. 



D. jS Arietis, in focus. 



The photographs of the spectrum of a Aquilae which have been obtained at 

 Kensington since 1890 were nearly all taken by Messrs. Fowler, Baxandall, 

 Shackleton, and North. Mr. Baxandall has assisted in the determination of 

 origins. 



The photographic plate has been prepared from the original negatives by 

 Sapper Wilkie, E.E. 



" The Velocity of tlie Ions produced in Gases by Eontgen Eays." 

 By John Zeleny, B.Sc, B.A., Assistant Professor of Physics, 

 University of Minnesota. Communicated by Professor J. J. 

 Thomson, P.RS. Eeceived February 15, — Eead March 1, 

 1900. 



(Abstract.) 



The sum of the velocities with which the positive and the negative 

 ions that are produced in gases by the Eontgen rays move when in a 

 unit electric field has already been determined by an indirect method 

 by E. Eutherf ord.* In the experiments here described the velocity 

 was determined in a number of gases for the positive and negative 

 ions separately, by comparing the ionic velocity directly with that of 

 a stream of gas. The stream of gas was made to flow between two 

 concentric cylinders, which were maintained at different potentials. 

 By passing a narrow beam of Eontgen rays through the cylinders at 

 right angles to their length, a narrow layer of ionised gas was pro- 

 duced. Due to the electric field between the two cylinders, the ions 

 of this layer tended to move radially towards, or away from, the axis 

 of the cylinders, but at the same time they were carried along by the 

 stream of gas. Of the ions of this layer which travelled inwards, 

 those that started from the inner surface of the outer cylinder were 

 carried a distance X by the gas stream before they reached the surface 

 of the inner cylinder. 



This distance is dependent directly upon the mean velocity of the 

 gas stream, and inversely upon the difference of potential between 

 the two cylinders. For obtaining the difference of potential which 

 must be used to allow the ions to be carried a certain distance along 

 the tubes by the gas stream the inner cylinder was divided at some 

 distance from the beam of rays into two parts, insulated from each 



* E. Eutherford, * Phil. Mag.,' November, 1897. 



