752 Potential required to maintain a Current in a Gas. 
at the positive electrode. The polarization in the layers of 
gas near the electrodes gradually rises, and eventually the 
normal anode and cathode falls of potential are established. 
Among other results, the experiments show that a current, 
accompanied by a glow in the gas, may be maintained when 
the fall of potential in the neighbourhood of the cathode is a 
small fraction of the normal cathode fall of potential. 
These results show that some of the recent theories which 
have been proposed to explain the maintenance of a current 
are greatly at fault. 
The phenomena which accompany the discharge of elec- 
tricity in vacuum-tubes have usually been studied with small 
electrodes and large currents, so that experimenters have 
concluded that the cathode fall of potential is a constant and 
independent of the current. 
It has been suggested that the normal cathode fall of potential 
is the potential required to impart a sufficient velocity to the 
positive ions in order that they may produce new ions on 
colliding with the cathode. If such a potential were required, 
it would be difficult to explain the maintenance of a current 
in cases where the fall of potential in the given layer near 
the cathode is much smaller than the cathode fall of 
potential. | 
A method has been given by Professor Thomson (J. J. 
Thomson, ‘ Conduction of Electricity through Gases,’ p. 381) 
of calculating the spark-potential by finding the falls of 
potential along the different portions of the distanee between 
the electrodes when the gas is carrying a current and the 
normal cathode fall of potential is developed. It is difficult 
to see why the potential so calculated should be the sparking 
potential, as it should represent the potential required to 
maintain a current which may be several hundreds of volts 
less than the sparking potential. Also, the calculated 
potential is independent of the current, whereas experiments 
have shown that the potential required to maintain a current 
depends on the value of the current as well as on the pressure 
of the gas and the distance between the electrodes. 
This theory, therefore, does not give a satisfactory expla- 
nation of these potentials, as there is no distinction drawn 
between the sparking potential and the potential required to 
maintain a continuous current. 
