X-RAYS: A SKETCH OF THEIR HISTORY. . 
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_ as for ordinary bulbs by an induction coil or high-tension transformer. The 
earliest forms of bulb were very similar in size and shape to those used for 
the earlier type of X-ray tube. In later types of Coolidge tube considerable 
modifications have been introduced. i . 
In order that the highest degree of vacuum—the air remaining after 
exhaustion must be less than one hundred millionth of that occupying the 
bulb at atmospheric pressure—may be not only attained but also maintained 
under the most severe conditions of operation, the bulb and its internal metal 
fittings are subjected to an extraordinarily thorough process of treatment and 
evacuation. The main difficulty in maintaining such an extremely high 
vacuum in a glass bulb containing metal parts arises from the circumstance 
. that both glass and metal occlude air and other gases. This occluded gas 
is not released, or is released-only in part by the mere process of evacuation, 
however thorough, but would be set free to a greater and greater extent as 
the tube becomes heated during operation, thus destroying the vacuum and 
making the tube inefficient, if not inoperable. To get rid of this occluded gas 
the bulb during the process of evacuation is contained within an electrically- 
heated oven maintained at a temperature of 360° C. or even higher, this high 
temperature having the effect of promoting the escape of occluded gas from 
the inner surface of the glass. After high vacuum has been attained the 
metal parts are intermittently subjected to bombardment by electrons and 
thus raised to a-bright red heat. It is for this reason that the metals tungsten 
and molybdenum are employed in construction, both of these metals having 
exceedingly high melting points. 
During the later stages of exhaustion the bulb is operated intermittently 
with increasing filament-current and increasing voltage between anode and 
kathode, the effect of this being tq raise the temperature of the massive 
tungsten target to a bright white heat. 
The action of the older types of X-ray bulb is accompanied by a bright 
green fluorescence of the glass. This phenomenon, in fact, is often looked 
upon as a criterion of good operation. It is, however, a secondary effect 
due to radiation from the residual gas, and hence is entirely absent in a good 
Coolidge tube. The operator who is in charge of the tube during the process 
of exhaustion can indeed gauge the progress toward its final stage by obsery- 
ing the gradual diminution of fluorescence of the wall of the bulb. 
When finally exhausted the vacuum is so good that no discharge can be 
forced through the bulb with the highest attainable voltages unless the 
filament is heated to a temperature at which it emits electrons; and unless 
the discharge is so heavy. as to actually volatilize the metal of the target 
the current is entirely undirectional. The Coolidge tube is thus a perfect 
_ rectifier of alternating current at any voltage. Inverse current, the béte noire 
of the ordinary bulb, is thus entirely eliminated. The primary and all- 
important advantage which the Coolidge or thermionic X-ray tube possesses 
over its predecessors, however, is the separate and precise controllability of 
the current through the bulb and of the voltage applied to the bulb. The 
former is controlled entirely by the temperature of the filament, which again 
depends on the current sent through the filament; the latter is controlled 
by the voltage across the terminals of the induction coil on high-tension 
transformer. It is thus possible to have either a very copious X-radiation - 
of low penetrating power or a relatively feeble one of very high penetrating 
power, a fact of the utmost importance both in radiography and in radio- 
therapy. In the older ionisation type of bulb, on the contrary, the radiation 
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