Time-Lao in the Spark Discharge. 229 



attempted, but the following figures show the nature of the 

 results with a hard gap of which the steady sparking potential 

 was 3120 volts. The first column gives the time taken by the 

 potential to rise to the steady sparking potential, the second 

 the actual sparking potential, the third the time-lag. 



0-02 sec. 3180 volt." 0*11 sec. 



0-008 3840 0*030 



0-0004 4080 0-0017 



< 0-00003 4540 < 0-00008 



It thus appears that the actual sparking potential is greater 

 the more rapid the rise of potential, but that the lag, and also the 

 whole time which elapses from the beginning of the rise to the 

 passage of the spark, decreases as the rate of rise increases. 

 In another series of experiments the rate of rise of potential 

 was kept constant, and the relation between the actual 

 sparking potential and the steady sparking potential deter- 

 mined as the latter was increased by increasing the sparking 

 distance. The results are given in the following table, in 

 which the first column gives the steady sparking potential, 

 the second the actual sparking potential, the last the excess 

 of the latter over the former. 



3090 volt. 4780 volt. 1690 volt. 



5230 6700 1470 



8040 8700 660 



These figures show that the hardness, measured by the 

 difference between actual and steady sparking potential, 

 decreases as the latter increases. The time-lag was difficult 

 to measure in this case because the rate of rise of potential 

 was so rapid; this rate decreased somewhat as the potential 

 rose, so that it is not certain that the time-lag decreased with 

 the spark potential, but it certainly did not increase. 



It is reasonable to suppose that the time-lag in these cases 

 consists of the period required to remove the film on the 

 cathode and to bare its surface for the liberation of electrons. 

 If this is so, all the facts may be simply interpreted in the 

 statement that the greater the excess of the potential above 

 the steady sparking potential, the more quickly is the film 

 removed. It is not pretended, however, that the phenomenon 

 has been thoroughly elucidated. Investigation of it is dif- 

 ficult because it is so variable. .It is difficult to be sure that 

 successive observations are made on a cathode in the same 

 condition. But the observations do suffice to show that 

 hardness due to surface condition is a phenomenon perfectly 

 distinct from the dependence on frequency of the sparking 

 potential of point gaps, and that it is due to a perfectly 

 different cause. 



