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The caliper method requires much calculation, and is subject to error. We have devised a slide rule that enables one to read the depth without calculation.
Metal pins are placed, like the rungs of a ladder, 1 cm. apart, and the ladder is placed perpendicularly between the table and the screen with the tube beneath the table, and the target 60 cm. from the screen. The tube is so placed under the ladder that the shadows of the pins are as but one in the central day; the screen is replaced by a plate, and a brief exposure is made with the diaphragm closed to admit only the central ray, to obtain 0. The tube is then shifted 12 cm. and a roentgenogram made on the same plate. This scale is transferred to the instrument illustrated, and thus the one minute depth measure is secured.
The point 12 cm. to
Kariher HC, Brown RD. AN IMPROVED CALIPER METHOD FOR ROENTGEN-RAY LOCALIZATION OF FOREIGN BODIES. JAMA. 1918;70(10):678. doi:https://doi.org/10.1001/jama.1918.26010100002009b
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