20 



THE ROYAL SOCIETY OF CANADA 

 II. Apparatus 



The apparatus used in making the measurements was the same 

 as that used by McLennan and Keys^ for measuring the mobilities 

 of ions in air at high pressures. It is shown in Figs. 1 and 2. AB was a 



Fig. L 



thick circular plate of brass about 8 cm. in diameter. GH was a circu- 

 lar brass plate 2 cm. in diameter, and EFKL was a guard plate sur- 

 rounding GH. The plate GH was held firmly in position by an amber 

 plug QX (see Fig. 2), with its lower face in the same plane as that of 

 the guard plate EL. The plates AB and EL were kept at a distance 

 of 1 cm. apart by ebonite supports, and the clearance between GH and 

 the guard plate EL was about one-half a millimetre. A leading-in 

 wire PR was attached to GH, and passing through the amber plug, 

 QX, it made electrical connection with one of the pairs of quadrants 

 of a Dolezalek electrometer. A leading-in wire attached to AB enabled 

 one with an earthed battery of storage cells to charge the plate AB 

 to any desired voltage. This ionisation chamber was placed in a 

 strong steel cylinder, shown in Fig. 2, which had a capacity of about 

 1-5 litres. The wires leading from the plates AB and GH were passed 

 through the walls of the cylinder, but carefully insulated from it. 

 The leading wires and the ebonite plugs were tapered where they 

 passed through the walls of the cylinder to withstand pressure without 

 leakage, and soft black wax was melted into depressions above the 

 tapered surfaces. This was found to be more resilient and satis- 

 factory under high pressure than the harder forms of wax. The 

 guard plate EL was electrically connected to the cylinder, and the 

 latter was earthed. 



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



