Quantity of X Rays.

The quantity of rays produced must be taken into con- sideration as well as the quality, and depends on a number of factors more or less inconstant. These factors jointly determine the amount of electricity passing through the tube, and in direct consequence determine the amount of X-radia- tion produced. They include voltage and amperage of current . supplied, periodicity and regulation of interruption, action of induction-coil, and nature of the X-ray tube. Most of these factors are under direct control of the operator, but in practice uniformity is difficult to maintain, and even with careful regulation variations occur. Also, as will be con-

sidered later, the X-ray tube may alter considerably in nature during a single operation.

Thus it is very difficult, from any or all of those initial factors, to estimate quantity of X-radiation, and time standards of exposure based upon them as data are unreliable. For long exposures for therapeutic purposes they may, indeed, be dangerous.

Only by noting the actual current passing at any moment through a tube in action, or by noting the actual effect, chemical or otherwise, of the.rays produced, can one judge the quantity of X rays, and calculate their probable effect. None of our present methods of measuring the quantity of radiation or its effect are wholly satisfactory.

By watching the register of a milliamperemeter inserted in the tube circuit, the production of rays can for ordinary purposes be best measured, and the time of exposure for any desired effect judged. If this record tends to vary during the operation a time average may be taken, or by regulation of the current supplied to the induction-coil the amount passing through the tube may be kept constant. Since from a soft tube a relatively greater quantity of rays is produced, it is of importance to observe any softening of the tube during exposure. This is of special importance in therapeutic work, since therapeutic action seems to depend for its intensity directly on the quantity of radiation in the exposure. This change, corresponding to a fall of electric resistance in the tube, will be indicated by a rise in the reading of the milli- amperemeter, and, conversely, hardening of the tube will be indicated by a fall in the reading.

Where a maximum therapeutic effect is desired at one exposure, as for epilation in treatment of ringworm, it is pre-eminently important that the degree of exposure should be carefully measured, for the margin of safety between epilation and a serious dermatitis is narrow. For this purpose Sabouraud’s pastilles are very serviceable, though by no means deserving to be considered as a final standard. These are exposed to the rays during the actual exposure, and by chemical change directly measure the quantity of radiation and indicate the probable therapeutic effect. Con

fiisting of platino-cyanide of barium, thickly coated on small discs of paper, the pastilles on exposure to X rays alter in colour from a canary-yellow to a brown. The change is pro- portional to the cumulative effect of the exposure. Thus, by comparison with a standard tint the operator can determine from the pastille when the desired degree of exposure has been reached. The conditions to be observed in using these pastilles will be found described in the section dealing with therapeutics.

At the meeting of the British Medical Association at Exeter a meter was described which registered the electrolytic effect of the current passing in the tube circuit. The effect is, of course, cumulative, so that the amount of gas liberated and measured in a audiometer tube will bear a direct ratio to the current which has effected its disassociation. This is a very ingenious device, and well merits attention; but, for maxi- mum exposures, we feel that we should like to understand more fully the vagaries of X-ray tubes before we depend on any other measurement than that directly made of the actinium effect of the radiation in question.

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