Gases at Reduced Pressures by Transient Arcs. 451 



maintained arcs. This is more noticeable when the inductance 

 of the circuit is low ; when it is high there is a flaming arc 

 which, though it has a resistance low compared with that 

 of the film of gas at the cathode, obscures the effect. 



The ignition of gases by such noninductive transient 

 arc- between platinum ' poles is remarkably uniform in type, 

 differing in this from the more varied phenomena of impulsive 

 spark ignition. It is in one sense simpler than the latter in 

 that there is no preliminary ionization of the gas before the 

 spark passes. The action is therefore confined to contact 

 between the explosive mixture and hot metallic vapour in 

 which there is intense electrification under the combined 

 influence of high temperature and strong field. The mode 

 of ignition is then a chemical combination of the gases 

 started partly by high temperature collision, as in flame, 

 aided by the direct electrification of the incandescent vapour 

 with which the gases collide. At normal pressures heat 

 alone may be more important ; at low pressures the tempera- 

 ture of the arc falls, but ionization by collision rises in 

 value — the electric gradient being kept the same. 



Ic was therefore to be expected that the limiting con- 

 ditions of ignition would vary and show critical points or 

 phases, characteristic respectively of heat and ionization, 

 though not to the same extent as with impulsive discharge, 

 and the results obtained show this to be the case. 



2. The nature of Transient Arcs. 



The voltage across the points of break of a circuit is the 



di 

 sum of the inductive voltage L-r and of part of the circuit 



& dt 1 



volts, the remainder being that absorbed in the resistance. 

 In the present work the inductance was made as low as 

 possible by the use of flat woven grid resistances. Oscillo- 

 grams of break reveal no voltage overshoot, the circuit break 

 sparks were therefore miniature arcs extinguished by air 

 cooling. 



The influence of gas pressure on the properties of such 

 arcs is now being investigated. It lias been found that as 

 the pressure is lowered the length of the arc falls to a 

 minimum, almost to zero, after which the discharge changes 

 character, spreads into a luminous glow, lengthens and 

 increases in duration. This change is always exceedingly 

 sudden, and is possibly in the first place to be associated 

 with the sudden drop in the number of ions emitted from a 



