and Ohmic Resistance of Gases. 361 



The evidence of all these experiments is unequivocal, and 

 may be summed up under the following heads : — 



I. The resistance of a gas at low pressure to the oscillatory 



discharge is equivalent to only a very small ohmic 

 resistance. 



II. This resistance is in general greater the less the quantity 

 of electricity. 



III. Down to a very small pressure this resistance decreases 

 with the tension of the gas. At a pressure considerably 

 below the minima in the potential curves given on 

 p. 351, the resistance seems to reach a minimum, but 

 the irregularity of the sparks in this region makes this 

 last minimum somewhat uncertain. The minima of 

 resistance probably vary with the quantity of elec- 

 tricity discharged as well as with the specific properties 

 of the gas. The minimum for nitrogen is attained at 

 a much lower pressure than that of hydrogen. 



IV. The form of the tube has an important effect upon the 

 resistance of the gas. 



V. With the oscillatory discharge it is evident that the 



electrodes produce far less effect than with the continuous 



discharge. 

 These conclusions are not wholly without support in the 

 literature of the subject. J. J. Thomson's researches with 

 tubes without electrodes * show that a rarefied gas must have 

 an extremely low resistance to the oscillatory discharge of 

 electricity, and Jaumann f found that the " electric strength " 

 of a gas increased as the quantity of electricity diminished. 

 But all earlier accurate measurements have been made with 

 continuous currents, whose relations are very different from 

 those of the sudden discharge of a condenser which we have 

 studied. The continuous current meets with great opposition, 

 especially at the negative electrodes, where much heat is 

 developed. The oscillatory discharge meets with very little 

 opposition, and correspondingly we find that here the greatest 

 heat is developed in the gas itself, especially in the capillary 

 tube, the electrodes remaining comparatively cool. This 

 experiment we have tried repeatedly, sending exactly equal 

 amounts of electricity through the tube in each fashion, and 

 observing great differences in the heating effects. Moreover, 

 even in the continuously glowing gas itself apart from the 

 electrodes, the potential-difference, if due to resistance alone, 

 corresponds to vastly greater resistance than that opposed to 



* i Recent Researches in Electricity and Magnetism/ p. 92. 

 t Sitz.-berichte Wien. Akad. vol. xcrii. p. 765. 



