452 Prof. W. M. Thornton on the Ignition of 



hot surface when the temperature reaches a critical value*. 

 The electronic state is set up at values X/p from '2 to *5, 

 where X is in volts per centimetre and p millimetres of 

 mercury. In order to have this at pressures of say 300 

 millimetres in air on arcs 5 mm. long X must be from 60 to 

 150, or the voltage on the arc 30 to 75. It is clear that this 

 will be passed through sometime during the break, for the 

 gap voltage starts at zero and ends at that o£ the circuit, in 

 this case 100. 



3. Results with, Direct Current. 



The break was made electromagnetically between platinum 

 rods by drawing an iron plunger, to which they were 

 attached, into the core of a coilf- The curves obtained, 

 figs. 1 to 4, have for ordinates the currents broken, and for 

 abscissae the pressures in atmospheres. They all approach a 

 lower limit at a little above 0*2 atmosphere, and pass through 

 an extraordinary fluctuation of inflammability from 0*4 to 

 0*5 atmosphere, after which they fall smoothly to a minimum 

 reached in most cases at about 1/5 atmospheres. 



The most interesting feature of this set of curves is the 

 progressive change of the paraffins as the order rises. 

 Hydrogen is in a class by itself ; the currents are much 

 smaller and the oscillation greater. Regarding the latter as 

 an indication of instability about a mean ordinate similar to 

 that of the curves of chancre of state, the following values of 

 the ordinafes are taken to give equal areas above and below 

 the mean line : 



Hydrogen 0'30 ampere. 



Methane 1*20 



Ethane 1-12 



Propane 0*87 „ 



Pentane 0*66 ,, 



For ethane, propane and pentane, these currents are 



23'5 



expressed by z = 0-334h = ^-p- , that is, in the limit 



mol weight 



the higher paraffins approach hydrogen in ease of ignition. 



* J. J. Thomson: 'The Conduction of 'Electricity in Gases,' 

 pp. 479-480. 



t " The Influence of Pressure on the Ignition of Methane." Brit. 

 Assoc. Newcastle; 'The Electrician,' Sept. 8th, 1916. 



