THEOKY OF ELECTKIC DISCHARGES 67 



which is the equation that determines the sparking 

 potential. Hence the sparking potential may be denned 

 as the potential which is required to maintain a very 

 small current in the gas. 



27. Currents accompanied by a positive charge in the gas. 



In order to investigate the effect of the increase 

 of current on the potential, it is necessary to consider 

 how the force between the plates is affected by the 

 electricity in the~gas. The positive ions, being of larger 

 mass, travel more slowly than the negative ions, so that 

 there is a greater number of positive than of negative ions 

 in the gas at any time. This excess of positive electricity 

 causes an appreciable disturbance in the uniformity of 

 the field as the current rises, and since all the positive 

 ions must pass through the gas at the negative electrode, 

 the positive charge is greatest in the neighbourhood of 

 that electrode. The effect of this charge is to increase 

 the force near the negative electrode and to diminish it 

 in other parts of the field. The ionizing power of the 

 positive ions is much increased in the field of strong 

 electric force, and when that field is near the negative 

 electrode the supply of ions is kept up when the potential 

 difference between the electrodes is less than the sparking 

 potential. 



Experiments on currents through rarefied gases in 

 discharge tubes show that the force is greater near the 

 cathode than at other points of the discharge. The 

 phenomena which attend these discharges are compli- 

 cated, and have given rise to numerous speculations, but 

 the principal variation in the electric force was first 

 explained by Professor Schuster 1 in the manner given 



1 Bakerian Lecture, Proc. Hoy. Soc., xlvii., 1890, p. 541. 



F 2 



