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THE APPARATUS 25

A Gaede pump worked by a motor is connected with the
discharge tube, and keeps the pressure in this part of the ap-
paratus at a suitable value. When the rays in some particular
gas are under examination a constant stream of this gas is
kept flowing through the discharge tube. The gas is stored
in the vessel A, Fig. 13, over a column of mercury : this vessel
is connected with the discharge tube by the system shown in
Fig. 13, where EC is an exceedingly fine capillary tube.
When the tap T is turned the gas has to pass through this
capillary: it does so exceedingly slowly. The rate can be

adjusted by raising or lowering a mercury reservoir connected
with A, this is held in such a position that when the Gaede
pump is in action the pressure in the discharge tube is such as
to give well developed positive rays. To screen off the magnetic
field due to the electromagnet, thick iron plates V,W, Fig. 10,
are placed round the neck of the tube.

The curves on the photographic plates made by the
positive particles are measured by the apparatus represented
in Fig. 14. The photographic plate is clamped in a holder A,
and the position of any point on it is determined by moving



	THE APPARATUS 25 A Gaede pump worked by a motor is connected with the discharge tube, and keeps the pressure in this part of the ap- paratus at a suitable value. When the rays in some particular gas are under examination a constant stream of this gas is kept flowing through the discharge tube. The gas is stored in the vessel A, Fig. 13, over a column of mercury : this vessel is connected with the discharge tube by the system shown in Fig. 13, where EC is an exceedingly fine capillary tube. When the tap T is turned the gas has to pass through this capillary: it does so exceedingly slowly. The rate can be



	adjusted by raising or lowering a mercury reservoir connected with A, this is held in such a position that when the Gaede pump is in action the pressure in the discharge tube is such as to give well developed positive rays. To screen off the magnetic field due to the electromagnet, thick iron plates V,W, Fig. 10, are placed round the neck of the tube. The curves on the photographic plates made by the positive particles are measured by the apparatus represented in Fig. 14. The photographic plate is clamped in a holder A, and the position of any point on it is determined by moving