270 Electric Strength of Mixtures of Nitrogen, fyc. [June 21. 



hydrogen is increased. With spark length 0'047 in. it varies from, 

 about 11 mm. in pure hydrogen to about 5 mm. in pure nitrogen. 



At high pressures the E.M.F. required to produce a discharge at a 

 given pressure diminishes continually as the proportion of hydrogen 

 to nitrogen is increased. At low pressures the relation between the 

 composition of the mixture and the E.M.F. required to produce a spark 

 at a given pressure is less simple. It is represented by the curves 

 in fig. 8, where the ordinates give the E.M.F. in volts, and the ratio 

 in which the ordinate divides the line NH is equal to the ratio of the 

 volumes of nitrogen and hydrogen in the mixture. 



FIG, 8. 



600 



500 



400 



Ni:0 



0:iH 



The curves are given for pressures of 20, 15, 10, 5, and 4 mm. 

 They could not be considered accurate for lower pressures, owing to 

 the extreme steepness of the curves (1 7), in consequence of which 

 the E.M.F., which will produce a spark at a given low pressure, 

 cannot be determined with any precision. 



It will be seen that at the pressures 15 and 20 mm. the general 

 slope of the curves is downwards from the nitrogen end to the 

 hydrogen end, but that at pressures 10, 5, 4 the slope is the other 

 way, showing that at low pressures the effect of introducing more 

 hydrogen is in general to increase the electric strength of the 

 mixture. 



But as the proportion of nitrogen to hydrogen increases from : 1 

 to 1 : 0, the electric strength does not diminish uniformly, but it 

 may pass through one or more maxima and minima. 



It is, perhaps, hardly necessary to say that the curves in fig. 8 

 c-annot be regarded as accurate for strengths of mixture intermediate 

 between those at which the observations were actually made. The t 



