370 Dr. E. Goldstein on the Electric 



kathode-light at which the currents of the inductorium either 

 cease to pass through the exhausted tube or else pass no longer 

 through the micrometer*. 



* The distances of the halls of the spark-micrometer corresponding to 

 these two alternatives are not exactly identical. The discharges do not 

 pass exclusively through the exhausted tuhe up to a certain distance of 

 the halls, and then with a certain small decrease of this distance exclu- 

 sively across the air-space between the balls ; but between the distances 

 of the balls at which the spark takes one only of the two courses open to it 

 are to be found positions of the micrometer at which both paths are taken 

 — sometimes the one, sometimes the other — and the one path the less fre- 

 quently the nearer we come to the distance at which the other is taken 

 alone. This apparently unstable character of the resistance in the tube 

 does not affect the accuracy of the measurements here in question ; we 

 may compare the resistances by comparing the distances of the balls 

 apart at which for a certain fixed time, say two minutes, no spark passes 

 in the tube or between the poles of the micrometer. The distances so 

 measured agree upon repetition to J s millim. — that is to say, to a fraction 

 of 1 per cent, of the distance measured. The alternation of the discharges 

 with certain distances of the balls no doubt depends, partly at least, upon 

 the same cause as the following phenomenon, which I have observed in 

 the same experiments, and which, as far as I know, has not been pre- 

 viously described. 



If we include the spark-micrometer in a branch circuit of an exhausted 

 tube which transmits both the discharge at " make" and that at " break," 

 then, if the distance between the balls of the micrometer be gradually 

 diminished, the current at "break " completely leaves the tube and passes 

 altogether in the free air, whilst that at " make " continues to traverse the 

 tube with undiminished light. Consequently we are able to study the 

 discharge of the " make " current in the exhausted gas separate from that 

 of the " break " current ; whereas hitherto a separation of the two currents 

 has been effected by introducing an air-break in the direct circuit inclu- 

 ding the tube, with which arrangement the tube is traversed only by the 

 discharge due to the "break" current. The reason of this phenomenon 

 may lie in the different maximum tension of the current on " making " 

 and "breaking" contact, and may correspond to the observation of tele- 

 graphists, that discharges of great tension (as observed in electrical 

 storms) will sooner traverse a short distance through air in the form of a 

 spark than traverse a long metallic circuit in the form of a current. 



Such phenomena show that in the case of discharge through gases, the 

 division of currents cannot be calculated according to Ohm's law. This 

 we see from the so-called Holtz's " funnel-tubes." If two similar tubes 

 are placed opposite to each other side by side in the same induced currrent, 

 then at suitable pressure of gas the current does not subdivide itself be- 

 tween the two in any definite ratio to the resistances of the tubes, but 

 the one tube remains entirely dark, the current goes altogether through 

 the other. 



The law according to which currents divide when the discharge takes 

 place in gas must therefore be investigated empirically in the first place. 

 I take this opportunity of remarking that I have made an erroneous 

 assumption on this subject in a series of experiments in my book pre- 

 viously referred to, which, however, does not actually affect the result 

 obtained. I believed myself justified in assuming as the evidence of 

 certain phenomena (p. 146), that if a part of the discharge traverse a me- 

 tallic circuit from the kathode a to an electrode b, and then the resistance 

 of a moistened thread be introduced between l and a wire c, that thr-n the 



