198 
MR. J. ZELENY ON THE VELOCITY OF THE IONS 
make electrical connections. The part B' was joined to earth, while the part B was 
connected to a pair of the quadrants of the electrometer, E. Great care was taken to 
adjust the position of the central cylinder so as to be accurately concentric with the 
outer one. 
The ends of the outer cylinder were fitted with the large rubber stoppers F and F'. 
Through these passed the gas inlet and outlet tubes, whose ends were the elongated 
funnels J and J'. These funnels, together with the cone endings of the inner 
cylinder, made the lines of gas motion change less abruptly on entering and leaving 
the apparatus, and so aided in having the gas maintain a steady motion in DD', where 
the observations were taken. At the left end, F, a rubber tube led to a gas bag of 
about 150 litres cajDacity. The manometer, I, measured the pressure of the gas in the 
apparatus. The right end, F', was connected to the glass wool chamber, G, which 
served to remove dust and any stray electrification from the gas. A rubber tube then 
led to a drying or moistening apparatus, to be described later, which was connected to 
a large gasometer of the ordinary type. The pressure of the gas in the gasometer 
was measured by means of a manometer, and a scale was also attached to the 
gasometer for measuring its rate of descent during an experiment. The average 
velocity of the gas stream in the apparatus was determined from the volume emitted 
by the gasometer in a second, and from the area of the cross section between the two 
cylinders. To prevent the gas in the gasometer from getting moist too rapidly in 
those cases where dry gases were used, the surface of the water was covered with a 
layer of oil, such as is used for air pumps, because of its very low vapour pressure. 
The board, XX', with the attached cylinders was placed on the top of a lead- 
covered box, UU', so that DD', the aluminium portion of the outer tube, was above 
the aluminium window, W, in the box. 
The box contained the Crookes’ tube and the induction coil for operating it. The 
form of tube used was that which the writer has previously employed for similar 
work. # This form was more satisfactory than any of the others tried, and gave the 
best results when emitting weak rays, and wdren an interval of rest of at least three 
or four minutes was allowed between the periods of use, which did not exceed thirty 
seconds. A 6-inch Apps’ coil was used with a hammer interrupter, which could be 
made to run with sufficient uniformity with an easy running weak ray tube. The 
source of the rays, T, was more than 20 centims. from the axis of the cylinders. 
The narrow vertical beam of rays which was sent up through the cylinders was 
regulated by adjusting the position of the tube, T, and of the lead plate, S, with its 
narrow slit, and of the two lead rings, L and Lj which fitted over the cylinder, DD'. 
This adjustment was first made by geometrical arrangement, and then tested and 
completed with the aid of a fluorescent screen placed over the apparatus. The lead 
strips, H and H', served to restrict the window, W, and the lead cover, Z, prevented 
any rays or ionized gas from reaching the outside air of the room. 
* J. Zeleny, ‘Phil. Mag.,’ July, 1898, p. 126. 
