350 Messrs. Trowbridge and Richards on the Temperature 



the oscillatory discharge of a condenser produces another. In 

 considering this question one is immediately struck by the fact 

 that, although the gas acts as if it presented a resistance of 

 several hundred thousand or even several million ohms to the 

 current, while under the influence of the continuous discharge, 

 nevertheless this same tube allows oscillations which are 

 wholly damped by a few hundred ohms to pass through it 

 under the influence of a condenser. These considerations led 

 us to measure the resistance of such a tube to the oscillatory 

 discharge, and we found by means of a novel method that 

 in fact a mass of gas at low tension contained in a capillary 

 tube may act as though it opposed a resistance of only five or six 

 ohms to the spark of a large condenser. 



In order the more clearly to grasp the situation, the 

 potential-differences between the ends of the tube during a 

 continuous discharge may w^ell be considered first. A 

 number of measurements of such potential-differences have 

 been made by Hittorf * and others, but it may be well to'give 

 two of the many series of measurements which we have 

 made, in order to facilitate comparison with the discharge of 

 the condenser through the same tubes. The tubes employed 

 throughout this research were of the ordinary type devised 

 by Plucker, consisting of two cylindrical bulbs separated by 

 a capillary 1*3 mm. in diameter and 7 cm. long. The 

 electrodes were of aluminium. Unless otherwise stated all 

 the experiments here recorded were made with tubes of 

 exactly this shape and size ; and most of the experiments 

 were made with a single tube. The voltmeter used for 

 measuring the potential-differences was a Thomson electro- 

 static electrometer, and the current used was not much over 

 a milliampere. 



As the voltmeter was only graduated to 1800 volts, the 

 readings above that amount are merely approximations. 



Each gas evidently has its minimum of potential-difference, 

 that of hydrogen lying at about 1 mm. of pressure, and that 

 of nitrogen at about 0*3 mm. These minima, as well as the 

 total potential-differences, are undoubtedly modified by the 

 strength of the current; but the results given above are com- 

 parable with one another because they were all made under 

 the same conditions. Hittorf found a minimum at about 

 0'35 mm. for nitrogen, and he pointed out by means of his 

 extra electrodes that. the fall of potential was very irregular, 

 the greater part of it residing at the kathode. His results 

 have been confirmed by others, and Wood t has shown that the 

 heat evolved at different parts of the tube follows the same 

 irregularities as these potential- differences. 



* Wied. Ann. xx. p. 705. t Ibid. lix. p. 238. 



