374 



Mr. W. E. Carr. On the Laws governing [Feb. 11, 



" On the Laws governing Electric Discharges in Gases at Low 

 Pressures." By W. E, Cake, B.A., University of Toronto. 

 Communicated by Professor J. J. Thomson, F.E.S. Eeceived 

 February 11, — Eead March 5 ; 1903. 



(Abstract.) 



The experiments described in this paper were undertaken with the 

 object of determining the potential difference required to produce 

 discharge in a number of gases over a wide range of pressures, and 

 especially of ascertaining if the law enunciated by Paschen* was 

 generally applicable, provided the electric field in which the discharge 

 took place was uniform. 



The paper is divided into the following sections : — 



(1.) Introduction. 



(2.) Description of apparatus. 



(3.) Experiments in air. 



(4.) Experiments in hydrogen. 



(5.) Experiments in carbon dioxide. 



(6.) Spark potentials with different electrodes. 



(7.) Minimum spark potentials. 



(8.) Connection between spark lengths and spark potentials. 

 (9.) Minimum spark potentials in different gases. 

 (10.) Summary of results. 



Paschen's experiments showed that when a given potential difference 

 was applied to two spherical electrodes whose distance apart could 

 be varied, the maximum pressure at which discharge occurred in a gas 

 varied inversely with the distance between the electrodes. The range 

 of pressures covered by his experiments did not extend below 2 cm. 

 of mercury. 



While Paschen has shown that as the pressure of a gas diminishes the 

 difference of potential necessary to produce discharge between electrodes 

 in a gas, a fixed distance apart, also diminishes, Peacef has shown 

 that a critical pressure is finally reached when the spark potential 

 reaches a minimum value, and that below this critical pressure the 

 potential difference required to produce discharge rapidly increases as 

 the pressure is lowered. Peace's experiments were conducted with air, 

 and his electrodes consisted of a pair of large parallel plates supported 

 in the gas. The values of the spark potentials recorded by him led to 

 the conclusion that Paschen's law did not hold for electric discharges 

 at and below the critical pressure. 



In this paper it is shown that with the apparatus used by Peace, the 



* Paschen, ' Wied. Aim.,' vol. 37, 1889, p. 79. 

 t Peace, ' Roy. Soc. Proc.,' vol. 52, p. 99. 



