Absorption of X-flai/f 



475 



the two pencils of rays are allowed to penetrate, become 

 equally strong. By combining the ionization-chambers with 

 each other so as to make the two saturation currents 

 neutralize each other, the null-point method can be 

 used. After determining the thickness of the water sheet 

 the relation between the absorption coefficients of the 

 element examined and of water is easily found. The experi- 

 mental arrangements are more plainly made out by the 



Fig.l. 



♦Z20-^-| 



schematic figure (fig. 1) above. The two ionization-chambers 

 (A and B), consisting of metallic boxes, in which as elec- 

 trodes, insulated from the boxes by sulphur, metal sheets are 

 fixed, and each of which is supplied with a mica window 

 (2*5 cm. diam.), are, by way of avoiding influence from the 

 bulb and from the high-tension wires, placed in a large 

 earthed metal box (C), the front of which is a lead screen 

 (D). Just in front of the said mica windows, apertures are 

 made in the lead screen, and before these, there are two 

 absorption vessels (E and JF) at a distance of 4 cm. from 

 the respective ionization-chambers. In one of these are the 

 parallel walls, bounding the liquid sheet to be examined, 

 at a distance of 1 cm. from each other, and made of thin 

 mica, whereas the other walls are made of glass, The vessel 



2 L 2 



