THE APPARATUS 21
glass flask A : a volume of from one to two litres is a convenient
size for this purpose. The cathode C is placed In the neck of
the flask. The position of the front of the cathode has a very
considerable influence on the brightness of the positive rays and
ought to be carefully attended to. The best position seems
to be when the front of the cathode is flush with the prolonga-
tion of the wider portion of the flask. The shape of the
cathode is represented in section in Fig. 11 : the face of the
cathode is made of aluminium, the other portion is soft iron.
A hole is bored right through the cathode to admit the fine
tube through which the positive rays are to pass. Care should
be taken to bore this hole so that its axis is the axis of sym-
metry of the cathode. The tube through which the positive
size for this purpose. The cathode C is placed In the neck of
the flask. The position of the front of the cathode has a very
considerable influence on the brightness of the positive rays and
ought to be carefully attended to. The best position seems
to be when the front of the cathode is flush with the prolonga-
tion of the wider portion of the flask. The shape of the
cathode is represented in section in Fig. 11 : the face of the
cathode is made of aluminium, the other portion is soft iron.
A hole is bored right through the cathode to admit the fine
tube through which the positive rays are to pass. Care should
be taken to bore this hole so that its axis is the axis of sym-
metry of the cathode. The tube through which the positive
FIG. ii.
rays pass is fastened into the cathode in the way shown in
Fig. ii.
Fig. ii.
The bore of this tube will vary with the object of the ex-
periment If very accurate measurements are required, the
diameter of the tube must be reduced to -I mm. or less. With
these very fine tubes, however, very long exposures (i-J to
2 hours) are necessary. The length of the tube is about 7 cm.
The tubes are prepared by drawing out very fine bore copper
tubing until the bore is reduced to the desired size. The tube
is straightened by rolling it between two plane surfaces, and
great care must be taken to get the tube accurately straight,
as the most frequent cause of dimness in the positive rays is
the crookedness of the tube. After long use the end of the
tube nearest the discharge tube gets pulverized by the impact
of the positive rays, and the metallic dust sometimes silts up
periment If very accurate measurements are required, the
diameter of the tube must be reduced to -I mm. or less. With
these very fine tubes, however, very long exposures (i-J to
2 hours) are necessary. The length of the tube is about 7 cm.
The tubes are prepared by drawing out very fine bore copper
tubing until the bore is reduced to the desired size. The tube
is straightened by rolling it between two plane surfaces, and
great care must be taken to get the tube accurately straight,
as the most frequent cause of dimness in the positive rays is
the crookedness of the tube. After long use the end of the
tube nearest the discharge tube gets pulverized by the impact
of the positive rays, and the metallic dust sometimes silts up