22 THE ROYAL SOCIETY OF CANADA 



however, was any conductivity observed. The sensitivity of the electro- 

 meter, it may be stated, was such that currents of the order of 10~* 

 e.s.u. could have been measured. 



From this result it would appear that at a pressure such as that 

 used, the helium does not exhibit any conductivity other than that 

 which might be due to ionisation by the earth's penetrating radiation. 

 To measure the latter with the electrometer referred to, it would of 

 course have been necessary to use a larger volume of the gas between 

 the plates AB and GH. 



V. Mobility Measurements 



In view of the absence of anything in the nature of metallic 

 conduction as indicated by the experiment described above, it was 

 decided to make some measurements on the mobilities of the ions in 

 helium at the same high pressure, with the object of seeing whether 

 with this gas these magnitudes followed the inverse pressure law. 

 The method used was the same as that adopted by McLennan and 

 Keys^ in measuring the mobilities of ions in air at high pressures. 



The arrangement of the parts of the apparatus was the same as 

 that described above, with the addition of a polonium-coated, copper 

 plate CD, about 2-5 cm. in diameter, which was fitted into a recess 

 made in the plate AB, so that the polonium-coated surface was flush 

 with the surface of the latter. The chamber was then filled with 

 helium, purified in the same manner as that described for the pre- 

 ceding experiments. 



As the range in helium of the alpha particles from polonium has 

 been found by T. S. Taylor^ to be 16-70 cm. at atmospheric pressure, 

 the range at the pressure of 81 atmospheres would be, assuming the 

 inverse pressure law to hold, 0-21 cm. By the arrangement indicated, 

 therefore, an intense ionisation would be produced close to the polon- 

 ium-coated surface, and confined to a layer of the gas about 2 mm. in 

 thickness. 



By applying various voltages between the plates AB and GH, 

 and measuring the current passing to the plate GH due to the passage 

 of the ions, the mobilities of the ions, assuming the ionisation to be 

 confined to a very thin layer, can be found from the formula given 

 by Rutherfordl 



(1) K = ' cm /sec. per 300 volts /cm. 



i and V being expressed in e.s.u. 



' McLennan and Keys. Phil. Mag., vol. XXX. Oct. 1915. 



2 Taylor. Phil. Mag., vol. XXVI, p. 402 (1913). 



3 Rutherford. Phys. Review. Vol. XIII, p. 321 (1901). 



