rents 
r 
May 14, 1914] 
NATURE 
271 
obtained. Moreover, the kathode itself must 
carry a rninute trace of gas to facilitate the trans- 
fer of electricity from the metal to the surround- 
ing space. Further, the great heat often gener- 
ated at the antikathode raises the temperature 
of the walls to such an extent that gas is set free 
there, and the balance of working conditions 
upset. A new tube, in fact, generally requires 
“maturing ” until its vacuum when at work will 
keep practically 
constant, and yet 
be neither too 
high nor too 
low. This calls 
for much care 
and patience on 
the part of the 
operator. A 
bulb may in this 
way be coaxed to 
Garry) 5 milli- 
amperes for 
bawrs at a 
stretch, and be 
of great service 
ime. treatment.” 
In most cases, 
however, the 
current does not 
exceed 2. milli- 
amperes for that 
purpose. But in 
radiographic 
work the usual 
practice is to 
employ a heavy 
current—2o milliamperes—for a few seconds or 
even a fraction of a second. The length of ex- 
posure is, of course, determined by a number of 
considerations, but with such a large current it 
cannot exceed a few seconds on account of the 
enormous heat generated at the antikathode by 
the impact of electrons. 
The same difficulty is met with in the treatment 
tubes, quite apart from the disengagement of gas, 
egrets 
NS 
Fic. 2.—Heavy-discharge X-ray tube. 
Fic. 3-—Heavy discharge radiator tube. 
for a longer exposure has to be given now than 
would be necessary if more current could be 
carried with regularity and safety. 
Under existing conditions the antikathode stem 
is usually made hollow, so as to enable water 
or air to flow in and carry off the heat, or an 
actual radiator may be fitted, as shown in Fig. 3 
(Cossor). The osmosis regulator is also repre- 
sented in the same illustration. Otherwise, even 
WOutge ea. VOL. O3\ 
the hardest substance used as target would melt 
or become pitted by the impinging electrons. As 
an example of what occurs, the photomicrograph 
(Fig. 4; for which I am indebted to Dr. Rodman) 
of a plate of platinoid-nickel which has served as 
target may be of interest. In all cases except for 
very light work, the antikathode is made of stout 
copper faced with platinum-iridium or pure iridium, 
tantalum, etc., at the place where the kathode 
rays impinge upon it. The uncertain variation of 
vacuum, together with the development of exces- 
sive heat at the antikathode, constitute the most 
serious objections to the present system of work- 
ing. The first difficulty especially hinders pro- 
gress towards the attainment of an accurate 
method of measuring or describing the radiation 
dealt with, for it may change from day to day, 
or even during an exposure. But there are, in 
addition, many minor ailments which develop with 
the age of the tube. Thus a deposit comes gradu- 
ally upon the inner surface of the walls. It is 
biG. 4.—Photomicrograph of eroded target. 
mainly metallic, and occludes gas; but it also 
provides electrified areas which disturb the normal 
streams of electrons. Occasionally, too, patches 
of bright fluorescence appear on the glass, due to 
specks of foreign matter sticking to the kathode. 
The direction of the discharge will sometimes 
reverse in the tube from no apparent cause. In- 
deed, the behaviour of a bulb is so erratic at 
times that a superstitious person might be ex- 
_cused for regarding it with distrust. 
However, with care and experience, and in 
spite of many disadvantages, splendid work is 
being done with this super-sensitive apparatus. 
But it is none the less necessary to make every 
effort towards placing X-ray therapeutics upon an 
accurate quantitative basis, and to simplify the 
technique. With existing appliances the prospect 
of so doing is remote indeed. 
But we are on the eve of great improvements ! 
Dr. Lilienfeld, of Leipzig, has already constructed 
a Réntgen tube which is so highly exhausted that 
the residual gas plays no part in the working 
