224 Dr. Norman Campbell on 



potential along the high resistance. Accordingly the fact 

 that, if the potential were raised to its maximum more 

 quickly, the spark did sometimes pass indicates that in these 

 cases the corona did not develop fully. The full corona 

 cannot be a necessary forerunner of the spark, but it is quite 

 conceivable that some initial condition which may develop 

 either into the corona or into the spark is such a necessary 

 forerunner. 



Theory of the regular lag. 



And it is not difficult to suggest what this condition may be. 

 Attention has so far been concentrated on the fact that there 

 is a lag, regular and irregular, in the needle gap : but from 

 one point of view it is much more remarkable that there is 

 no time-lag in the sphere gap, even when the frequency is as 

 great as a million a second and the sparking distance as great 

 as 10 cm. For if, as the theory of casual ions suggests, the 

 discharge is started at one point in the gup and spreads 

 thence to the remainder, it would be expected that, apart 

 from the irregular lag, there should be a regular lag equal 

 to the period required for the ions to spread from the 

 starting-point throughout the whole gap. ]S T ow the sparking 

 field is about 30,000 volts per cm.; the velocity of the ions 

 about 1*5 cm. /(sec. volt); and the time required for the 

 ions to spread over a gap of 10 cm. should be of the order of 

 10" 4 sec* How then can a spark occur when the whole 

 duration of the potential is little more than 10~ 6 sec. ? 



The answer is, of course, that the discharge does not start 

 in one place only in a sphere gap. It starts at a great many 

 places at the same time, and the lag which must occur is 

 only the time required for the ions to travel from one of these 

 places to the other. But if this is so, the difference between 

 the sphere and needle gaps is at once explained ; for in 

 the needle gap the field midway between the needles is 

 usually much too small to cause ionization by collision. The 

 discharge cannot be initiated there ; it has to spread to such 

 regions from the neighbourhood of the electrodes where the 

 field is more intense. It is the time required for this 

 spreading which is doubtless the regular time-lag ; it will 

 increase, as is shown by Peek's observations, with the 

 distance between the electrodes ; it will increase also with 

 the humidity which reduces the speed of the ions. 



* This estimate might be reduced about 100 times if it is supposed 

 that the ions concerned are electrons which remain free the whole time. 

 But, according- to the accepted theory, the spark discharge cannot stait 

 without ionization by the positive ions, so that these must have time to 

 travel through the whole gap. 



