Maximum Frequency of X Rays from a Coolidge Tube. 345 



Determination of the Absorption Curves, 



The general arrangement of the apparatus for this purpose 

 is shown in fig. 2. The X rays, passing through a rect- 

 angular opening (6x6 cm.) in the lead box, entered the 



Fig. 2. 



ionization chamber A. This consisted of a lead box 

 (15x15x15 cm.) 3 mm. thick, divided into two equal 

 partitions by the electrode B. The two insulated plates 

 C and D were generally connected together and charged to 

 a P.D. of 1000 volts, which was found to be sufficient to 

 produce saturation under the experimental conditions. Two 

 equal openings were cut in the front face, which were closed 

 with a thin sheet of mica to make the vessel air-tight, and 

 covered with thin aluminium foil to make it conducting. 

 Two thick lead slides, SS, were constructed to control the 

 width of the beam of rays entering either half of the ioniza- 

 tion vessel. The current supplied to the electrode B was 

 measured by a Lutz string electrometer, which proved very 

 suitable in all respects for this purpose. The instrument 

 was very easily set up, and sufficient sensibility was obtained 

 when the plates were charged to ±15 volts, while the zero- 

 was very steady and the deflexions read with ease. The 

 quartz fibre was broken during the course of the experi- 

 ments, and the later measurements were made with a Kaye- 

 Wilson electroscope. 



The main difficulty in these experiments lay in the 

 fluctuations of the current through the bulb, and consequent 

 variation in the intensity of the radiation. To correct for 

 this, the radiation before entering the ionization chamber A 

 passed through a u standardizing vessel M V of thin parallel 



