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22 RAYS OF POSITIVE ELECTRICITY

the tube and prevents the rays getting through. The cathode
is fastened In the glass vessel by a little sealing-wax, and a
similar joint unites it to the ebonite box, UV. To keep the
joints cool and prevent any vapour coming from the wax, the
joints are surrounded by a water jacket J through which a
stream of cold water circulates.

The electric ield is produced between the faces of L and M
which are pieces of soft Iron with plane faces. These are fitted
Into the ebonite box UV so that their faces are parallel: the
distance between the faces should be small compared with their
lengths. In many of the experiments described subsequently
the length of the faces was 3 cm. and their distance apart
i"5 mm. Their faces are connected with the terminals of a
battery of small storage cells: In this way any required differ-
ence of potential can be maintained between them.

These pieces of soft Iron practically form the poles of an
electromagnetic, for the poles of the electromagnet P and Q
are made of soft Iron of the same cross section as L,M ; they
fit into Indentations In the outside of the ebonite box and are
only separated from the pieces L,M, by the thin flat pieces of
ebonite which form the walls of the box. This arrangement
makes the magnetic field as nearly coterminous as possible with
the electric, which Is desirable in several of the experiments.
A conical glass vessel F 40 cm. long is fastened by wax to
the ebonite box while the other end is fixed to the apparatus
which contains the photographic plate. One form of this, de-
signed by Mr. Aston, Is represented in'Fig. 12. The photo-
graphic plate Is suspended by a silk thread wound round a tap
T which fits into a ground glass joint; by turning the tap the
thread can be rolled or unrolled and the plate lifted up or let
down. The plate slides in a vertical box B made of thin metal;
this Is light tight except at the openings A which are placed
$0 that the positive rays can pass through them. The open-



	22 RAYS OF POSITIVE ELECTRICITY the tube and prevents the rays getting through. The cathode is fastened In the glass vessel by a little sealing-wax, and a similar joint unites it to the ebonite box, UV. To keep the joints cool and prevent any vapour coming from the wax, the joints are surrounded by a water jacket J through which a stream of cold water circulates. The electric ield is produced between the faces of L and M which are pieces of soft Iron with plane faces. These are fitted Into the ebonite box UV so that their faces are parallel: the distance between the faces should be small compared with their lengths. In many of the experiments described subsequently the length of the faces was 3 cm. and their distance apart i"5 mm. Their faces are connected with the terminals of a battery of small storage cells: In this way any required differ- ence of potential can be maintained between them. These pieces of soft Iron practically form the poles of an electromagnetic, for the poles of the electromagnet P and Q are made of soft Iron of the same cross section as L,M ; they fit into Indentations In the outside of the ebonite box and are only separated from the pieces L,M, by the thin flat pieces of ebonite which form the walls of the box. This arrangement makes the magnetic field as nearly coterminous as possible with the electric, which Is desirable in several of the experiments. A conical glass vessel F 40 cm. long is fastened by wax to the ebonite box while the other end is fixed to the apparatus which contains the photographic plate. One form of this, de- signed by Mr. Aston, Is represented in'Fig. 12. The photo- graphic plate Is suspended by a silk thread wound round a tap T which fits into a ground glass joint; by turning the tap the thread can be rolled or unrolled and the plate lifted up or let down. The plate slides in a vertical box B made of thin metal; this Is light tight except at the openings A which are placed $0 that the positive rays can pass through them. The open-