X-Rays, Apparatus and Methods Page 10

number of breaks. Indeed, it is often far from being so. Some of the best X-ray pictures that have been taken have been by the use of the old "hammer" break of Ritchie, and 4 or 5 clicks of the break have sufficed. Using the same tube and coil, but a rapidly vibrating or rotary break, required 40 or 50 breaks to accomplish the same results. It would appear, hence, that the energy of X-ray radiation per break is a function of the number of breaks per unit of time.

We are inclined to believe that this is a matter purely of the "time constant" of the coil. The iron of the core requires a certain time to magnetize and demagnetize. With the breaks few, ample time is given for this process, but when the breaks become rapid this is no longer possible, so that the secondary E.M.F. falls—this may easily be verified by observing the maximum spark length of any coil — with single breaks, few and far between, we get much longer spark maxima than if the breaks are frequent. The thickness of the sparks is also greatly increased. If, however, we increase the primary E.M.F. for the more rapid break we may bring up the secondary spark length to the value given by the single breaks. We should thus be able to shorten the exposure as much as we please, by merely increasing the rate of break and the primary E.M.F. at the same time; but here we are limited by the tube which cannot dissipate more than a certain definite amount of energy in a given time and will break down if overdriven. We see, therefore, that whether we use slow breaks or fast breaks, the actual time of exposure remains practically the same, the few breaks requiring to be distributed over the same time as the much larger number of rapid breaks. All this, of course, applies to the photographic plate. For fluoroscopy we require rapid breaks in order to escape the otherwise distressing flickering.

In the above connection, particular attention is directed to one point — the rate of break must not be too great. If it is, the iron no longer has time to magnetize and demagnetize. The result is an alternating secondary discharge instead of the direct discharge desired, and this means blackening of