Electrical Conductivity in Gases traversed hy Cathode Hays. 377: 



the same time as the cathode rays. The results of experiment, how- 

 ever, are entirely opposed to this view, and lead to the conclusion that, 

 if any Kontgen rays are present in the cathode pencil, they must be of 

 so weak a character that their ionising action can be neglected. A 

 direct comparison showed the ionisation by cathode rays to be about 

 300 times that due to an intense Rontgen radiation. 



In the conductivity produced by cathode rays, the current of elec- 

 tricity does not increase in proportion to the electromotive force applied. 

 The current, after reaching a certain critical value, becomes practically 

 stationary and increases but little when very large increases are made 

 in the electric field. With Rontgen or uranium radiation fields of 400 

 or 500 volts a centimetre have sufficed to give saturation in the case of 

 most simple gases, but in the present investigation it was necessary to 

 go as high as 1000 volts a centimetre before the maximum current was 

 reached. 



In order to compare the ionisations in two different gases, or in the 

 same gas under different conditions, recomse was had to the use of two 

 ionising chambers. The discharge tube was provided with a double 

 cathode, and carried two aluminium windows. Two pencils of rays 

 were obtained in this way, whose intensities were found to maintain a 

 constant ratio, and these were used to produce the ionisations in the 

 two chambers. 



The ionisation in air, kept at a constant pressure in one of the 

 chambers, was taken as the standard. The gases, whose ionisations 

 were to be compared, were placed in turn in the other chamber, and 

 their conductivities, as measured by saturation currents, were found in 

 terms of the standard. 



An important result, obtained by this method with cathode rays of 

 constant intensity, was the agreement found to exist between the 

 ionisation in hydrogen at atmospheric pressure, and that in air at a 

 pressure of 53 mm. At these pressures the two gases had the same 

 density, and in both cases, therefore, according to Lenard's absorption 

 law, the disposition of the rays, their actual intensities, and the amount 

 of them absorbed from point to point in the ionising chamber, were 

 precisely the same. Under these conditions the equal ionisations 

 obtained not only form a strong confirmation of Lenard's absorption 

 law, but they also show that, where equal absorption of cathode rays 

 occurs, equal ionisation is produced. 



In order to test the conclusion more closely experiments were made 

 with air, hydrogen, carbon dioxide, oxygen, nitrogen, and nitrous 

 oxide, and in all cases it was found that, when these gases were re- 

 duced to the same density, the same ionisation was produced in them 

 by rays of constant intensity. 



It follows, therefore, that an ionisation law exists exactly analogous 

 to that of absorption, namely, that when cathode rays of a given 



