SOME EXPERIMENTS WITH KATHODE 
RA MSs 
THE extensive employment of the focus form of Crookes’ 
tubes as the most efficient known means of generating X- 
rays, has rendered advisable the more complete investigation of | 
the kathode ray discharge in tubes of this description. 
Hitherto, the usual method of investigating the characteristics 
of a kathode ray discharge apart from its mechanical properties, 
and beyond what is visible to the unassisted eye, has been by 
allowing the rays to fall upon a screen of some brightly fluorescent 
material, such as glasses of various descriptions, or screens 
covered with fluorescent salts. With all of these the maximum 
amount of fluorescence appears to be produced by such com- 
paratively weak kathode rays, that in some cases the special 
effects produced by the more powerful rays séem to be more or 
less entirely masked, while the well-known phenomenon of the 
fatigue of fluorescent substances, when exposed to the more 
active rays, conduces to the same result. 
Surface Luminescence of Carbon when exposed to Kathode 
Rays. 
I have found in some cases that by replacing the usual screen, 
made of or covered with fluorescent material, by one of ordinary 
electric light carbon, much appears 
which was previously invisible. When 
a concentrated stream of powerful 
kathode rays are focussed upon a sur- 
face of carbon in this manner, a very 
brilliant and distinctly defined lumin- 
escent spot appears on the surface of 
the carbon at the point of impact of 
the rays, the remainder of the carbon 
remaining black. This luminescent spot 
seems to have a very close relation to 
the fluorescent spots on glass and on 
other fluorescent materials under similar 
influence. The effect is evidently a 
purely surface effect, as when the 
kathode stream is rapidly deflected by 
means of a magnet, the luminescent 
spot on the carbon moves with no per- 
ceptible lag. Further, though, as is 
also the case with glass, the whole of 
the carbon becomes gradually heated 
to a considerable extent if much power 
be employed for a long period of time, 
these luminescent spots are instantane- 
ously produced on carbon of very con- 
siderable brilliancy with but a compara- 
tively low power. Again, just as glass 
is known to become fatigued under the 
influence of kathode rays, so that after 
a time it refuses to fluoresce so brightly 
as before, so carbon is similarly fatigued, 
though only after having been very 
strongly acted upon. Carbon, like glass, 
also recovers its property of giving a 
surface luminescence to some extent, 
though it does not seem to entirely recover, 
all rapidly. 
atsany rate, at 
Apparent form of the Kathode Ray Discharge in a Focus 
Tube. 
As is well known, in tubes of the ordinary focus type with a 
single spherical concave kathode, the rays coming off normally 
to the kathode surface appear to converge in more or less of a 
cone to a focus, and if the vacuum be not too high, to diverge 
again immediately in another cone upon the other side of the 
focus. At higher vacua the rays, after passing the focus, do not 
appear to diverge again at once, but seem to form themselves 
into a description of thread which connects the convergent and 
divergent cones, and is longer or shorter according as the vacuum 
is higher or lower. The focus, or perhaps more correctly, the 
point at which this thread commences, seems always to be more 
distant from the kathode than the centre of curvature of the 
latter, but the variation in this respect seems to be less and less 
the higher the exhaustion. This is no doubt due to the mutual 
repulsion of the rays, and accords with the assumption that the 
1 Abstract of a paper by A. A. C. Swinton, read before the Royal Society, 
March 11. 
NO. 1443,) Ola 55 
NATURE 
[| APRIL 15, 1897 
rays consist of charged particles, which travel more and more 
rapidly the higher the exhaustion. Probably for the same reason, 
kathodes that are only slightly concave, focus further in pro- 
portion beyond their centres of curvature than do deeply concave 
kathodes, for the same vacuum. 
Apparent Hollowness of the Divergent and Conve 
Rays. 
‘gent Cones of 
When the divergent cone is thrown upon a thin platinum disc, 
as in the ordinary focus tube, and sufficient electric power—say, 
from a 10-in. Ruhmkorff coil—is employed, the platinum quickly 
attains toa red heat. With platinum, either the whole disc be- 
comes uniformly heated, or in the event of the diameter of the 
cone of rays where it strikes the platinum being small compared 
with the area of the platinum, that portion of the platinum 
covered by the base of the cone becomes uniformly heated to a 
higher temperature than the remainder. This is as much as can 
usually be seen with platinum, though rather more is sometimes 
visible with aluminium ; but if instead of either metal the disc 
is made of ordinary electric-light carbon, I have found that the 
luminescent portion of the carbon, instead of comprising the whole 
disc, or consisting of a uniformly heated circle, will in some cases 
take the shape of a brilliantly luminescent and apparently white 
Fics. r- 
4e 
hot ring, with a well-defined dark, and seemingly quite cold, 
interior. As the dimensions of the cone of rays are increased or 
decreased by decreasing or increasing the vacuum, the lumin- 
escent ring will be found to increase or decrease correspondingly 
in diameter, at the same time being brighter when small than 
when large. Further, when the ring is very small it will usually 
have a very brightly luminescent central spot, with a dark inter- 
vening portion between this spot and the ring, and when the 
vacuum is further increased the ring will gradually close in upon 
the spot until only the latter remains. 
Figs. 1, 2, 3, and 4 show diagrammatically these hollow effects 
for four different degrees of vacuum, 1 being the lowest and 4 the 
highest exhaustion. The upper portion of each of these figures 
represents the general appearance of the kathode discharge between 
the spherical concave aluminium kathode C at the top, and the 
carbon anti-kathode B at the bottom. Beneath each of the 
elevational views of the kathode discharge will be found a plan 
view of the carbon anti-kathode, showing for each condition of 
vacuum the effect of the kathode discharge upon the carbon anti- 
kathode, in forming a brightly luminescent hollow ring, gradu- 
ally decreasing in diameter as the vacuum is increased, untll it 
centres on a point, as already mentioned. 
