SPARKING POTENTIAL IN A UNIFORM ELECTRIC FIELD 53 



is known as Paschen's 1 law, and it has been found by 

 Carr 2 to hold for large ranges of low pressures in the 

 neighbourhood of the critical pressure. 



All the experimental results at different distances and 

 pressures may therefore be represented by a single curve, 

 whose ordinates are the sparking potential and the pro- 

 duct of the pressure and distance between the plates. In 

 particular, it will be noticed that the critical pressure 

 corresponding to the minimum sparking potential is 

 inversely proportional to the distance between the plates. 



21. Potential required to maintain a discharge. 



When a spark discharge takes place between parallel 

 plate electrodes the gas immediately becomes a con- 

 ductor, and allows a comparatively large current to pass 

 through. When the pressure is greater than the critical 

 pressure the potential required to maintain the current 

 is very much less than the sparking potential, and the 

 potential difference between the electrodes diminishes as 

 the current increases. When the pressure is less than "; 

 the critical pressure a small current is produced when the 

 sparking potential is applied, and in order to increase ? 

 the current it is necessary to increase the potential 

 difference between the electrodes. 3 



The potential required to maintain a current may be 

 represented by the dotted line P' Q K'. At a point B, 

 where the pressure is greater than the critical pressure, 

 the gas insulates until the potential is equal to E B. If 

 this potential be applied by a battery of cells through 

 a resistance, immediately the spark occurs a large current 



1 Paschen, Wied. Ann., xxxvii., 1889. 



2 W. E. Carr, Philosophical Transactions A, Vol. cci., pp. 403 

 433, 1903. 



8 J. A. Brown, Philosophical Magazine, September, 1906, 



