424 
ME. W, E. CAEE ON THE LAWS C40YEENING 
The curve B, corresponding to a pressure of 1000 millims., which is the critical 
pressure for the spark length of 5 micra, fig. 8, is a straight line and shows that the 
spark lengths are directly proportional to the spark potentials for the whole range of 
spark lengths. It will ])e noticed, too, that at this pressure the minimum spark 
potential, 350 volts, to which special attention has been drawn in this paper, is that 
Fig'. 9. 
potential necessary to lireak down tlie gas for the sliortest spark lengtli to which we 
have considered Paschen’s law is applicable. 
Again, the curve L), which is typical of all the curves for pressures below 
1000 millims. of mercury, expresses tlie relation between spark potentials and spark 
lengths for a pressure of 320 millims. 
It shows tliat the latio of sjiark potential to sparking distance; is constant for all 
spark lengths gieater than 15 or 16 micra. For shorter spark lengths the spark 
potential incieases with decreasing spark lengths until finally the 5-micra line is 
reached at a potential of about 820 volts. 
I he cuive A, wliich is a tyjie of those which can be drawn for pressures above 
1000 millims. of mercury, difiers from B in but one feature. The law of proportionality 
again holds throughout for this pressure down to the shortest spark length, lint it 
