Reduced Pressures by Impulsive Electric Sparks. 347 

 4. Pressure and Temperature in the Spark. 



The temperature of a disruptive spark, like that of an arc, 

 is probably not the same throughout. Towards the positive 

 pole the spark is brighter. This brightness falls off as the 

 pressure is reduced and eventually forms a glow corre- 

 sponding to the positive column in a vacuum-tube. In a 

 steady arc the heat is greatest at the positive pole, and the 

 temperature is at most that of vaporization of the electrode. 

 The observation* that the first spark of an oscillatory series 

 passes through air and the succeeding sparks through the 

 vapour of the metal of the electrodes, indicates that this 

 initial spark has a higher voltage gradient and tempe- 

 rature than those following. A higher temperature would 

 to some extent explain the fact, quoted in § 1, that this 

 first spark has greater power of ignition than the others ; 

 it certainly has a higher voltage gradient. When the 

 circuit is inductive the metal lines appear in the spectrum 

 of the first spark also. Apart from energy, all records of 

 ignition show that the discharge of a condenser with relatively 

 low inductance in series has greater activity as a source of 

 ignition than a single jump spark from an induction-coil. 



From photographs of spark discharge the actual break- 

 down which constitutes a disruptive spark takes place along 

 a localized narrow chain of ions. The volume of the spark 

 as such does not appear to be inversely proportional to 

 the pressure but is more nearly constant. Thus the lower 

 the pressure the fewer the number of molecules affected. 

 Haschek and Mache's conclusion j- that, pressures of 60 to 

 120 atmospheres occur in induction-coil spark discharge can 

 be taken to indicate transient temperatures higher than 

 those of the steady carbon arc. The pressures in condenser 

 discharge sparks have been estimated X to approach 1000 

 atmospheres, and these sparks pit platinum freely, single 

 induction-coil discharge does not. The temperatures in dis- 

 ruptive spark discharge are in any case much higher than 

 are required for ignition by a steady source of heat. 



At pressures lower than atmospheric the rise of pressure 

 in the spark and the temperature of discharge fall steadily. 

 If ignition depended upon temperature alone it should 

 increase in difficulty as the pressure is reduced until a 

 temperature is reached where a gas could not transmit flame. 



When ignition is by hot wires, inflammation is obtained at 

 pressures as low as 10 centimetres of mercury, or 0*13 of an 



* Schuster and Elemsalech, Phil. Trans, vol. cxciii. p. 189(1899). 



t Wied. Ann. lxviii. p. 740 (1899). 



X Sir J. J. Thomson, ' Conduction through Gases,' p. 517. 



2 A2 



