SCIENCE AND INDUSTRY. 
can be increased in quantity only by increasing the voltage applied between 
anode and kathode, with consequent increase in the hardness of the radiation, 
and, moreover, the voltage between the electrodes is limited by the amount 
of gas remaining in the bulb. The currents actually used in practice with 
Coolidge bulbs may range from 1 to 100. milli-amperes, the voltages from 
20,000 to 100,000 volts. Of course, the extreme values both of current and ~ 
voltage are used only in special circumstances; but the limitation to magnitude 
of current results rather from the volatilization of the kathode which would 
ensue were the tube operated for any length of time with currents of the 
order of 100 milli-amperes than from an insufficiency of electron supply 
from the hot kathode. The space-charge effect may also limit the current, 
particularly at low voltages. 
It. is possible by using a hollow target kept cool by a continual flow of 
» water through its interior to obtain outputs very greatly exceeding the 
values just mentioned. Water-cooling, however, though possible in an 
experimental laboratory, is troublesome and uncertain in practical radio- 
graphy, and ina recent type of bulb Dr. Coolidge attains the same end of dissi- 
pating the heat energy due to the bombardment of the anti-kathode by 
_ attaching it to a massive copper stem through which the heat is conducted 
to an external radiator. The anti-kathode itself consists of a tungsten 
button embedded in copper. The size of the bulb can consequently be 
greatly reduced, the latest type being approximately 2 inches in diameter 
and 10 inchesin length. The Coolidge bulb has other and not unimportant 
advantages over the ionisation bulb, given a suitable generator of high- 
tension currents. Its wide range in performance enables it to be applied 
- with equal success to a wide variety of cases in radiography or radio-therapy. 
It shows none of the capriciousness of the ionisation bulb which is so often 
the despair of its operator, and it will stand up to far heavier and more 
continuous work than will the ionisation bulb. 
How well it has commended itself to the medical profession is shown by 
the fact that from the first sale in 1913 to August, 1919, no less than 25,000 
of the bulbs had been sold by the General Electric Company. 
Though the application of X-rays to radiography of the human body 
for the purpose of medical diagnosis and to radio-therapy still remains and 
may perhaps always remain supreme in practical importance, a number of 
other fields of application are coming to light. Firstly, radiography of 
materials and structures is being utilized to a continually increasing extent. 
Thus the detection of flaws, blow-holes, &c. in castings or weldings of not 
too great thickness is possible by radiography. The accompanying illus- 
trations (Higs. 8 and 9) show better than words can do the measure. 
of success attained. It may be stated that for thicknesses up to 1 inch 
the method is both practicable and satisfactory. For greater thicknesses 
success will depend upon the nature of the specimen and of the flaws 
to be examined on the one hand and on the voltage which can be applied 
to the bulb on the other. Although Sir Robert Hadfield, the well-known 
English ironmaster, a pioneer and enthusiast in this field, predicts that it 
will eventually be possible to satisfactorily examine castings up to 9 inches 
in diameter, it must be pointed out that the difficulties both in operating a 
bulb at the high voltage necessary to secure the requisite penetration for 
“such large thicknesses of iron, and of securing photographs which are not 
completely “fogged” by the radiation scattered by such a mass of 
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