PRODUCED IN GASES BY RONTGEN RAYS. 
199 
The quadrant electrometer, E, used for making the measurements was a small 
bicellular one, the needle of which was suspended by a quartz fibre, and charged 
through the liquid below by means of a battery of 160 small storage cells. One 
pair of its quadrants w T as joined by a wire to the part BC of the inner cylinder. 
Both the electrometer and the connecting wire were surrounded by an earthed 
metal case. 
The key, K, permitted the insulated quadrants to be connected to earth at any 
time. 
The capacity of the two quadrants and the part of the inner cylinder connected to 
them, together with the connecting wire, was about 53 centims. The sensibility of 
the electrometer was about 500 divisions per volt, with the scale at a distance of 
130 centims. The potential of the outer cylinder AA' was maintained at any desired 
value by means of the battery of storage cells, JSI ; the arrangement of the extra 
cell, O, and the divided megohm, M, permitting the addition of a fractional part of a 
cell’s voltage. 
By opening a stop-cock on the gasometer the gas was made to pass from the 
gasometer, through the apparatus, into the gas bag on the other side, at a rate which 
was regulated by the weights on the gasometer. It could then be forced back into 
the gasometer and used again. 
A large volume of gas is required for carrying out an experiment, and the method 
is therefore limited to a small number of gases that can be obtained in such quantities, 
and that do not act upon the materials of the apparatus. 
§ 4. Corrections and Precautions Observed in the Experiments. 
1. It is essential for these experiments that in its motion down that part of the 
tube where the observations are being taken, the different portions of the gas should 
move in paths parallel to the axis of the tube, i.e ., that the motion be uniform, and 
not turbulent with vortices. This condition depends upon the velocity of the gas 
stream. 
O. Reynolds has shown* that for motion in a cylindrical tube a fluid when 
started in a turbulent state will tend to assume a uniform motion with the parts 
moving parallel to the axis when for the fluid the average velocity is less than a 
critical value, 
where p. is the viscosity of the fluid relative to that of water at 0°, p is its density, 
D is the diameter of the cylinder, and B is a constant. 
The value of B obtained was about 280 when D and V were measured in metres. 
Applying this constant to the gases used, for a cylinder of the diameter of the 
* O. Reynolds, 1 Phil. Trans.,’ A, 1883. 
