
Marcu 17, 1923] 
light rays radiate from a lamp. The X-ray tube is, 
_ indeed, an X-ray lamp in which the applied voltage 
is analogous to the temperature of a luminous lamp. 
If we raise the temperature of the latter, we shorten 
the average wave-length; so with the X-ray bulb, 
if we raise the voltage the average wave-length is 
diminished. 
In the light of present-day knowledge, what do we 
know to be the factors which control output from an 
X-ray tube? The radiographer is concerned more 
with the general or continuous spectrum of X-rays 
than with the superposed rays characteristic of 
the target. In regard to the former the factors 
are three: (a) The number of the cathode rays or 
electrons which strike the target ; (b) the speed of 
the electrons ; (c) the massiveness of the atom of the 
target. (a) is represented by the current through the 
tube, (b) by the voltage across the tube, (c) by the 
atomic weight or, more precisely, the atomic number 
_ of the metal of the target. 
To what extent do these several features come in ? 
To settle these points let us call in the aid of the X-ray 
spectrometer and vary each of the factors one by one. 
The spectrometer spreads out into a continuous fan 
of rays all the various wave-lengths present, and tells 
us, moreover, the amount or intensity of each wave- 
length. So that if we plot wave-length against in- 
tensity, we get a curve which clearly reveals the 
composition of the beam. Furthermore, the area 
of the curve is a measure of the output. If we do this, 
we find that the several spectral curves show that 
the X-ray output is proportional to the current, to 
the atomic number of the target, and to the square 
of the voltage. 
We notice that the voltage comes in as a second- 
power term, and the importance of measuring voltage 
by the radiographer not sporadically but as a routine 
procedure day in and day out should be stressed. 
For the applied voltage has a dual importance: it 
not only dominantly affects the output, but it is the 
sole arbiter of quality or penetrability or wave-length. 
The time is approaching when we must gradually 
relinquish the use of the terms “ hard ” and “ soft ” 
X-rays and accustom ourselves to speak of wave- 
lengths. For example, in deep therapy we can say 
that the spectrum of rays employed lies between 
006 and o:2 A.U., the mean effective wave-length 
being about o-15 A.U. or less. The radiographer who 
uses point spark-gaps up to, say, 6 inches long employs 
a spectrum of rays ranging from about o-r2 to o-4 
A.U., the mean effective wave-length generally lying 
between o-2 and o-3 according to the filter and nature 
of the high potential generator. We might make a 
beginning by agreeing, for example, that “ hard” 
rays refer to rays with wave-lengths shorter than 
ot A.U., and that “soft” rays have wave-lengths 
longer than o-3 A.U., the intervening rays being of 
“ medium ” hardness. 
Let us consider the career of an electron in an 
X-ray tube impelled towards the target with a velocity 
which it owes to the applied voltage V.. The chances 
that that electron will ultimately come into suitable 
conflict with one or more atoms in the target and so 
generate X-rays are slight—about 1000 to 1. The 
energy of the electron is, in fact, much more likely 
No. 2785, VoL. I11] 
NATURE 

365 
to be frittered away as heat. Assuming that the 
unlikely happens, one of two things may occur: the 
electron may lose all its energy at one encounter or 
it may do so by instalments in a succession of cn- 
counters. In other words, if we agree to think of its 
energy in terms of the original driving voltage (V), 
then it may lose the whole of V in one step or do so 
in a number of steps. 
Now Planck’s quantum relation tells us that when- 
ever an electron has its speed altered the wave-length 
of the X-ray produced is inversely proportional to 
the energy given up by the electron; that is, to the 
equivalent loss of propelling voltage. It will be noted 
that no question arises of the nature of the target. 
To put it another way : 
) “ (eee of 
resulting X-ray 

( Loss of propelling 
voltage on electron ) =const. 
In those encounters where the whole of the energy 
of the electron is transferred in one fell swoop, the 
shortest-waved X-rays possible to that voltage will 
be generated. They will be accompanied by a variety 
of longer waves depending on the varied experience 
of other electrons, but always a short-waved limit 
is set by the magnitude of the full exciting voltage. 
We are led to appreciate a number of other results. 
It is seen at once why we do not get (as was once 
imagined) homogeneous X-rays when a tube is excited 
by constant potential, and where all the electrons 
reach the target with the same velocity. Neverthe- 
less, we should expect that the proportion of short 
waves would be greater with constant potential than 
it would be with fluctuating potential, the peak value 
of which is equal to that of the constant potential. 
Furthermore, from what is known of the effect of 
voltage on output, we should anticipate a greater 
X-ray output (and less heating of the target) with a 
constant voltage than when that voltage is diluted 
with lower voltages. Both these surmises as to the 
superior efficacy of constant potential are confirmed 
by the X-ray spectrometer. 
With reference to the existence of a minimum wave- 
length or boundary to every spectrum of general X- 
rays, this is fully borne out by spectrometer measure- 
ments and photographs. Numerically, Planck’s rela- 
tion becomes : 
Minimum wave- - Maximum) __. 
( length in A.U, ) ( voltage ) =12,350. 
This very simple relation provides us with a scale of 
quality which, if not perfect, is more exact than any 
which the radiologist has been in the habit of using. 
If we glance at typical spectral curves of X-ray emission, 
we see that they are not symmetrical—the centre 
of gravity of the curve is well towards the quantum 
limit —the shortest waves are the dominating ones, 
and still more so if the rays are subjected to normal 
type filtering. The mean effective wave-length of a 
spectrum of rays is seen to approximate to the wave- 
length of the peak of the curve; that is, the wave- 
length of maximum intensity. Now, there is some 
evidence that the ‘‘ peak’ wave-length is proportional 
to the limiting or quantum wave-length, and this fact 
enables us so to identify very fairly the quality of a 
mixed bundle of X-rays. No doubt something depends 
on the wave-form of the exciting potential, but the 
