. Radiography and radio-therapeutics . his distance may be employed when the exact size of anorgan hke the heart is desired ; a good average working distance is about2 feet for parts of average thickness. A slide rule enables us to find out the necessary exposure first scale contains figures for the distance between anti-cathode andplate, varying from 12 up to 200 cm. On the second scale, figures for thethickness of the object, varying from 2| up to 50 cm., will be found. On ESTIMATION OF THE EXPOSURE 133 the third scale is the penetrating power of the tube in Wehnelt units,
. Radiography and radio-therapeutics . his distance may be employed when the exact size of anorgan hke the heart is desired ; a good average working distance is about2 feet for parts of average thickness. A slide rule enables us to find out the necessary exposure first scale contains figures for the distance between anti-cathode andplate, varying from 12 up to 200 cm. On the second scale, figures for thethickness of the object, varying from 2| up to 50 cm., will be found. On ESTIMATION OF THE EXPOSURE 133 the third scale is the penetrating power of the tube in Wehnelt units, from2 up to 18 ; and the fourth scale contains the figures for the milhamperesused, and rises from 0*5 up to 50 milUamperes. By adjusting the two slides so that the figures for the distance, thickness,penetration, and current which are being used are opposite to one another,the index on the second slide points to the number of seconds required for theexposure, which is on the fifth scale, beginning ^^^th I and rising up to Fig. 116.—Slide rule. On this basis it is possible to set down an Exposure Table which shallbe of some practical utility to the beginner. There are of course so manyspecial conditions that come into the matter that it is not possible to laydown hard and fast rules. Actual experience with the outfit and tubesis essential, combined with the exercise of good judgment. The followingtable will therefore only be taken as a guide, remembering always thatif the tube be softer or the distance greater the exposure must be corre-spondingly increased. The exposure must also be increased for abnormalstoutness, and so on. Above all, each focus tube, no matter what its degreeof hardness, must be worked to just that extent which signifies maximumefficiency, neither under-running nor overstraining. This point is dealtwith fully in the chapter on Tubes. Distance Penetration. from plateto anti- 1 Wehnelt Benoist and cathode. Scale. Bauer. Seconds. Inches. Sk
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