MEASUREMENT OF X-KAYS AND RADIUM 



75 



(1 to 2 per cent) from this source a thimble chamber should not be cali- 

 brated nearer than 50 cm. from the tube target and preferably as much as 

 100 cm. It must likewise be assured by test that the X-ray beam is of 

 uniform intensity over an area of diameter somewhat larger than the 

 length of the thimble chamber. 



When it is desired to read the thimble chamber and standard chamber 

 ionization simultaneously, the two may be set at different distances from 

 the tube and in such an alignment as not to interfere with each other. 

 In such case, the validity of the inverse square law must be separately 

 tested for each chamber, in addition to insuring that the test beam is 

 uniform at position of each. The replacement method is usually 

 preferable. 



Fig. 18. — Condenser, thimble ionization chambers full size. (Sievert.) 



Correction must likewise be made for the air density in the thimble 

 chamber at the time of calibration. This correction is obtained from 

 the atmospheric temperature and pressure in the same manner as for the 

 standard chamber. 



Types of Thimble-chamber Roentgenometer s.—Thimhle-chaniher roent- 

 genometers may be divided into three groups, (a) electroscope type, (6) 

 condenser type, and (c) vacuum-tube type. Instruments of any type, 

 ordinarily read the total number of roentgens per given deflection or 

 indication, and by noting the time of deflection the X-ray intensity is 

 given in roentgens per minute. In the first two types the voltage sensi- 

 tivity of the electrometer may vary over different parts of the scale; 

 hence, for example, the number of roentgens may be different for the 

 first and last half of the scale. Since this does not change with quality, 

 a single correction curve may be determined by noting the time rate of 

 change of the indicator between successive scale positions. Most Ameri- 



