. Radiography and radio-therapeutics . tandardsshould have thick lead glass. The instruments described all estimate more or less accurately the hardness of the X-ray bulb. There are also instruments which measure the quantity of current actually passmg to the tube. These are useful m estimating the exposure necessary at particular times. Later, the exact method of combining all the factors required for the estimation of exposure will be described. For our present general purpose it is sufficient to state that there are instruments used to measure the current passing through the tube, the actua
. Radiography and radio-therapeutics . tandardsshould have thick lead glass. The instruments described all estimate more or less accurately the hardness of the X-ray bulb. There are also instruments which measure the quantity of current actually passmg to the tube. These are useful m estimating the exposure necessary at particular times. Later, the exact method of combining all the factors required for the estimation of exposure will be described. For our present general purpose it is sufficient to state that there are instruments used to measure the current passing through the tube, the actual quantity of which ..11 vary with the internal resistance of the tube. For example, a soft tube ^^n\\ allow say, 10 milhamperes to pass, whilst with the same primary current a much harder tube ^vill only allow, say, 1 milhampere to P^^«- Measurement of X-Rays by -^--^^^^^^meter is necessary for this purpose when radiographic exposures are |venand acts by estimating the quantity of current passmg through the X-ray. Fig. 85. —Milliamperemeter.(Siemens.) 94 RADIOGKAPHY tube. That shown is a Deprez-dArsonval moving coil instrument, and itis an advantage to have the zero in the centre of the scale for measuringpositive and negative currents. They do not show the actual currenttraversing the X-ray tubes, but its mean value, which can, however, betaken as a relative measure of the intensity of the rays. So long as thepointer remams stationary this indicates that the hardness of the tube isconstant. Should the tube, when in use, become harder, the pointer willmove towards zero. The instrument is as a rule specially constructed sothat it is impossible for sparking to occur inside, and the pointer should bewell damped. There are other instruments for measuring the quantity ofX-rays used in dosage. These are described more fully in the section onRadio-therapeutics. The table below gives the comparative values of the instruments mostfrequently used, namely : Bauer,
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