and Ohmie Resistance of Gases. 



351 



Potential- differences between Electrodes of 

 Spectrum tube. 



Hydrogen. 



Nitrogen. 



Pressure in mm. 



Voltage. 



Pressure in mm. 



Voltage. 



70 



60 



40 



3-5 



20 



1-5 



1-25 



115 



1-00 

 •70 

 •50 

 •13 



2600 (?) 

 2100 (?) 

 1900 

 1500 

 1340 

 1260 

 1220 

 1150 

 1100 

 1140 

 1220 

 very high. 



8-5 



5-0 



4-0 



3-0 



25 



1-7 



1-4 



1-2 



10 



0-7 



06 



0-5 



03 



0-25 



0-13 



0-06 



very high, 

 very high. 



2600 (?) 



2100 (?) 



1750 



1600 



1410 



1340 



1180 



1140 



1080 



1040 

 980 



1030 



1700 



28004- (?) 



Neglecting the factors of the potential-difference which 

 reside at the electrodes, the sum of which increase with the 

 exhaustion of the tube, we find that according to Hittorf 's 

 results the resistance of the gas itself steadily diminishes as 

 the exhaustion proceeds ; for example, with a current of two 

 milliamperes he found a fall of potential of about 120 volts 

 between two parts of the middle of the tube eight centimetres 

 apart, the tension of the nitrogen being 0'35 mm. When 

 the current was about one milliampere, and the tension of the 

 gas was only about O'OOl mm., the voltage sank to fifteen. 

 These two figures correspond to resistances of 60,000 ohms 

 and 15,000 ohms respectively, the resistance of the gas 

 diminishing as the pressure is decreased. Of course we have 

 no certainty as to how much of this opposition to the current 

 is due to true resistance, and how much to a kind of polariza- 

 tion, but it is convenient for present purposes to count it all 

 as resistance. 



In any case this opposition, if maintained, is far too 

 great to permit the passage of oscillations, even under the 

 most favourable conditions. In order to prove that the 

 opposition is not maintained, but is in fact broken down 

 by the spark, it was only necessary to photograph the dis- 

 charge with the help of a rapidly revolving mirror, after the 

 method of Feddersen. Unfortunately, the light in the tube 

 itself is too faint for direct instantaneous photography ; but 



2 E 2 



