528 Prof. J. J. Thomson on the Charge of Electricity 



scope is necessarily heavy to afford the requisite stiffness, the 

 length under measurement is limited. 



For the purpose of making the angular displacements visible 

 to an audience a magnifying arrangement is used, consisting 

 of a tilting mirror supported upon a tripod. Two legs of the 

 tripod are supported by a hole and slot carried upon the 

 vernier-plate, and the third leg is supported upon a plane 

 attached to the other main piece. 



The spot of light reflected from the mirror is caused to 

 move over a fixed graduated scale, and the angular displace- 

 ment is therebv made visible. 



LVII. On the Charge of Electricity carried by the Ions produced 

 by Rontgen Rays. By J. J. Thomson, M.A., F.R.S., Caven- 

 dish Professor of Experimental Physics, Cambridge* . 



THE following experiments were made in order to deter- 

 mine the magnitude of the charge of electricit}? carried 

 by the ions which are produced when Rontgen rays pass 

 through a gas. 



The theory of the method used is as follows : — By mea- 

 suring the current passing through a gas exposed to Rontgen 

 rays and acted upon by a known electromotive force, we 

 determine the value of the product nev, where n is the number 

 of ions in unit volume of the gas, e the charge on an ion, and 

 v the mean velocity of the positive and negative ions under 

 the electromotive force to which they are exposed. 



Mr. Rutherford (Phil. Mag. vol. xliv. p. 422, 1897) has 

 determined the value of v for a considerable number of gases ; 

 using these values, the measurement of the current through a 

 gas gives us the product ne ; hence if we can determine n, 

 we can deduce the value of e. 



The method I have employed to determine n is founded on 

 the discovery made by Mr. C. T. R. Wilson (Phil. Trans. A, 

 18i)7, p. 265) that when Rontgen rays pass through dust- 

 free air a cloud is produced by an expansion which is incap- 

 able of producing cloudy condensation when the gas is not 

 exposed to these rays. When a determinate expansion is 

 suddenly produced in dust-free air a definite and calculable 

 amount of water is deposited in consequence of the lowering 

 of the temperature of the air by adiabatic expansion. When 

 the gas is exposed to the rays the ions caused by the rays 

 seem to act as nuclei around which the water condenses. I 

 have shown (' Applications of Dynamics to Physics and Che- 

 mistry/ p. 1 64) that on a charged sphere of less than a certain 

 * Communicated by the Author. 



