312 Prof. W. L. Bragg and Messrs. James and Bosanquet : 



that it is the same for any one face and order, however the 

 crystal may be distorted, and is, in fact, a definite physical 

 quantity on which theoretical calculations may be based. 



3. The X-ray spectrometer is of the type devised by 

 W. H. Bragg, which has been described in former papers. 

 The ionization-chamber is filled with methyl bromide, and a 

 potential of 320 volts is applied between the outer walls of 

 the chamber, and the inner electrode which is connected to 

 the electrometer. This potential is sufficient to prevent 

 appreciable recombination of ions with the strongest ioniza- 

 tion produced. A Lutz-Edelmann string electrometer is 

 used to observe the charge communicated to the electrode, 

 its sensitivity being adjusted to about 100 divisions to a volt. 

 A null method is employed to measure the charge. The 

 inner rod of a small cylindrical condenser is connected to 

 the electrometer, the outer cylinder being raised to any 

 desired potential by a potential divider and battery. When 

 the crystal is swept through the reflecting angle, a charge is 

 communicated to the electrometer. This charge is neutral- 

 ized by adjusting the potential of the outer cylinder of the 

 condenser until the string in the electrometer returns to its 

 zero. The potential applied to the condenser is then pro- 

 portional to the total charge which has passed through the 

 ionization-chamber. 



4. It is necessary that the incident beam of rays should 

 remain constant in intensity. A Coolidge bulb is used, 

 in which the anticathode consists of a button of rhodium 

 embedded in a tungsten block. A large induction-coil and 

 Sanax break supply a current of 1*5 milliamperes at a 

 potential of about 50,000 volts. It is possible to keep the 

 intensity of the rays constant to within 2 or 3 per cent, 

 and with the Coolidge tube it is also possible to repeat the 

 conditions of the experiment on successive occasions in a 

 satisfactory manner. Such variations in intensity as do 

 occur are probably due to the irregular action of the break. 



5. Superimposed on the homogeneous rays, there is a 

 general radiation of all wave-lengths which is also reflected 

 by the crystal. In making a measurement of intensity it is 

 necessary to allow for this general radiation. When com- 

 paring the intensity for two faces or orders, a preliminary 

 survey is made in each case to enable a measurement to be 

 made of the effect of the general radiation. The chamber is 

 set at a series of angles over a range including the angle at 

 which it receives the homogeneous beam. At each position 

 of the chamber, the crystal is swept through the corre- 

 sponding reflecting angle and the total ionization measured. 



