Jlogal Jnstttution of (§xtat ISritain. 



WEEKLY EVENING MEETING, 



Friday, Jauiiarj 19, 1906. 



The Right Hon. The Earl of Rosse, K.P. B.A. D.C.L. LL.D. 

 D.Sc. F.R.S., Vice-President, in the Chair. 



Professor Joseph John Thomson, M.A. LL.D. D.Sc F.R.S. M.R.I., 



Professor of Natural Philosophy, R.I. 



Some Applications of the Theory of Electric Discharge to Spectroscopy. 



The luminosity produced by an electric current passing through a 

 gas at low pressure varies greatly in character, not only when we 

 alter the nature of the discharge — as, for example, when we pass 

 from the arc to the spark — but also in many cases at different 

 points of the same discharge. The luminosity may be of one colour 

 at one place and of a very different one at another ; a spectroscopic 

 examination shows the spectrum of the same gas often varies con- 

 siderably as we proceed along the line of discharge. As recent 

 experiments have thrown a considerable amount of light on the 

 processes going on in the different kinds of electrical discharge 

 and at different parts of the same discharge, the study of the con- 

 nection between the changes in the electrical effects and the changes 

 in the spectra might be expected to throw some Ught on the very 

 interesting question of the genesis of spectra. Many important 

 points can very conveniently be studied by the aid of Wehnelt's 

 method of producing the current. In this method the cathode is 

 a strip of platinum or a piece of platinum wire on which either a 

 little lime or barium oxide has been deposited. This when heated to 

 redness emits large supplies of corpuscles, and by altering the tem- 

 perature of the platinum very large variations in the current passing 

 through the tube and the potential difference between the electrodes 

 can be obtained. In our experiments the current has varied from a 

 small fraction of a milliampere to several amperes, and the potential 

 difference from a few volts to several hundred. 



The apparatus used is shown in Fig 1. AB is the platinum strip 

 with the lime on it ; a thermo-couple, a platinum and platinum-rhodium 

 junction, Avas fused to this strip and served to determine its tempera- 

 ture ; the strip was connected Avith the earth and Avas heated by a 

 current passing through the leads L, M; a rheostat was placed in series 

 with the heating current and by means of this the temperature could be 

 altered gradually. The anode was a platinum disc ; this AA^as connected 

 with the positive pole of a battery of storage cells, the negative pole 

 of Avhich was earthed ; to alloAV of gradual Aariations in the potential 

 Vol. XVIII. (No. luu) o 



