Gases at Reduced Pressures bu Transient Arcs. 455 



The reciprocal of the time of duration T of an arc is the 

 analogue of temperature, and the equation 



(<+£) (P-6)=C/T 



can ba used to express fig. 1 in general form, where a is not 

 the constant of § 4, but a function of T. 



The simplest ignition by a break spark is that in which 

 the energy of the spark necessary for ignition is inversely 

 proportional to the number of molecules which come into 

 contact with it in unit time. In that case pqY is constant, 

 where p is the gas pressure, V the circuit voltage, and q = iT 

 thp quantity passing while the spark lasts. Vis constant and 

 />iT = constant is the first equation of this kind of ignition, 

 or ip = CjT. There is a lower limit of pressure b at which 

 the flame cannot travel by conduction and i (p — 6) = C/T 

 rakes account of this. So far there is no action considered 

 but the heat energy of the spark. The influence of calorific 

 value per unit volume is included in the term p. 



Any action other than thermal, such as that caused by a 

 change in the number of ions emitted from the hot pole or 

 produced in a second from any cause, must be expressed by 

 a modification of the current. If such an effect occurs which 

 retards ignition the current required will rise, if it accelerates 

 ignition it will fall, more or less suddenly according as the 

 action develops gradually or with critical sharpness. 



The temperature of the arc without doubt falls with the 

 pressure, its brightness is less. The volume of the arc, as 

 shown by its photographic image, at first falls and then at 

 low pressures rises. The one action that is definitely electrical 

 which might increase the igniting power of a transient arc is a 

 greater production or diffusion of ions in the gas in contact 

 with it, that is, an increase in their velocity. Judging by the 

 colour of a spark in gases with distinctive coloration, such as 

 cyanogen, there is ireo penetration of the arc by the gas. 

 The suggestion now made is that in ignition the "activation " 

 agreed by chemists to be necessary for combination in gases 

 and thought by physicists to be a blend of ionization and 

 high temperature collision, becomes suddenly more intense. 

 The oscillation of fig. 1 is found in ignition by disruptive 

 discharge, where there is ionization by collision before a 

 spark can pass. It is not found in the same mixtures when 

 ignition is by hot wires*, where ionization by collision does 

 not occur. 



* Loc. cit. tig. 8. 



