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X-RAYS: A SKETCH OF THEIR HISTORY. 
7.€., of one single wave-length, but consist of groups of such radiations 
differing slightly in wave-length. Thus the K-radiation contains four such 
groups, the L a greater number. 
For elements of low atomic weight these radiations merge into ultra-violet 
or even into visible light. 
The great: interest of these results for the X-ray technician lies in the 
substitution of a precise and definitely measurable magnitude, the wave- 
length, fora relatively indefinite one, the ‘ hardness ” or penetrating power, 
and further in the possibility which is afforded of analyzing precisely the 
quality of the radiation emitted by the target, or anti-kathode. It is found, 
in fact, that the radiation from the target resembles the light emitted from 
an incandescent vapour, say, a mercury-vapour lamp, insomuch as a general 
radiation corresponding to “white” light is superposed upon the almost 
monochromatic radiation characteristic of the atomic species of the target 
corresponding to the “* bright-line ” spectrum of the metallic vapour. Fig. 5 
shows the distribution of energy in the spectrum of the X-radiation from a 
tungsten target operated with 110 kilovolts between the terminals of the 
tube. 
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FIG. 5.—Distribution of energy in the spectrum of the 
X-radiation from a tungsten target. (A. W. Hull Physical 
Review, 1917.) 
A simple and important relation has been found to exist between the 
shortest wave-length occurring in the radiation and the voltage applied to 
the bulb, viz.: that this wave-length decreases in exact proportion as the 
voltage increases. Similarly, unless the applied voltage reach the value 
corresponding to the wave-length of the characteristic radiation, that radiation 
will not be excited. As an instance, fhe K-radiation of tungsten requires an 
applied voltage of 67 kilovolts, the L-radiation about one-eighth of this, and 
so on. The long-known relation existing between the spark-gap of the coil 
and the “hardness” of the rays from a bulb is thus explained and made 
precise. ; 
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