. Radiography, X-ray therapeutics and radium therapy . pass, whilst withthe same primary current a much harder tube will only allow, say, 1 milli-ampere to pass. Measurement of X-Rays by Milliampere-meter.—A milliamperemeter is necessary forthis purpose when radiographic exposures aregiven, and acts by estimating the quantity ofcurrent passing through the X-ray tube. Thatshown is a Deprez-dArsonval moving coil in-strument, and it is an advantage to have the zeroin the centre of the scale for measuring positiveand negative currents. They do not show theactual current traversing the X-ray tubes,
. Radiography, X-ray therapeutics and radium therapy . pass, whilst withthe same primary current a much harder tube will only allow, say, 1 milli-ampere to pass. Measurement of X-Rays by Milliampere-meter.—A milliamperemeter is necessary forthis purpose when radiographic exposures aregiven, and acts by estimating the quantity ofcurrent passing through the X-ray tube. Thatshown is a Deprez-dArsonval moving coil in-strument, and it is an advantage to have the zeroin the centre of the scale for measuring positiveand negative currents. They do not show theactual current traversing the X-ray tubes, butits mean value, which can, however, be taken asa relative measure of the intensity of the long as the pointer remains stationary thisindicates that the hardness of the tube is con-stant. Should the tube, when in use, becomeharder, the pointer will move towards zero. The instrument is as a rulespecially constructed so that it is impossible for sparking to occur inside,and the pointer should be well damped. There are other instruments for. Fig. 60.—Milliamperemeter.(Siemens.) MEASUREMENT OF HARDNESS OF TUBES 61 measuring the quantity of X-rays used in dosage. These are describedmore fully in the section on Radio-therapeutics. The table below gives the comparative values of the instruments mostfrequently used, namely : Bauer, Wehnelt, Walter, and Benoist. Comparative Scale of the usual Instruments for measuring theHardness of Tubes soft medium hard Bauer .... 1 2 3 4 5 7-5 6 7 8 9 10 Wehnelt . 1-5 3 4-5 6 9 10-5 12 13-5 15 Walter 1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 Benoist 1 2 3 4 5 6 7 8 9 10 The ingenious method used for the control of the Coolidge tube appearsto be the perfect one for the estimation and control of the hardness of theX-ray given off from the bulb. The penetrating power of the X-rays is dependent upon the speed ofthe cathode stream, and the latter is varied by increasing or diminishingthe potential at the terminals of the tube. The provision of an ammet
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