722 Dr. S. J. Allen on a Bull Instrument 



Table II. 

 Sensitiveness 084. 



Balance Method. 



" Bate of Leak " Method. 



Time 



Balance- 



Intensity. 



Per 



Time 



Bate of 



Per 



in mins. 



point. 



cent. 



in ruins, 

 i 



Leak. 



cent. 







9-1 



6-7 



100 







6-66 



100 



1 54 



7-5 



31 



46 



2 



5-45 



96 



! 85 



67 



1-9 



27 



22 



3-90 



69 



98 



6-4 



14 



21 



59 



2-34 



41 



165 



5-9 



•9 



12 



81 

 105 

 164 



1-54 



1-01 



•60 



27 

 18 

 11 



Note. — The first of these readings 











was taken about 5 minutes after the 











first one by the balance method. 



The X-ray tube was placed about 88 cms. from the window 

 of the testing-vessel, the axis of this being 45° to the plane 

 of the target. The tube was operated by a 10-inch induction- 

 coil placed about a metre from the instrument. Under these 

 conditions and without any further screening, the needle of 

 the electrometer was perfectly steady, and readings could be 

 readily made of the ionization due to the X rays. 



The opening at the window had to be narrowed down to 

 about 4 millimetres, so that a balance could be made with 

 the present uranium standard. For strong X-ray tubes a 

 much stronger standard would be more convenient. 



The readings for an absorption test of the X rays are 

 shown in Table III. The results indicate at once the diversity 

 of rays sent out by the tube ; the soft rays for the first few- 

 layers beiifg cut down to 50 per cent, by about 2 millimetres 

 of aluminium, while the harder kind are only cut down to 

 half value by about 5 millimetres of iron. 



The small quantity of rays which penetrated through the 

 case of the electrometer quickly discharged the needle, so 

 that it was necessary to place a thick lead screen in front of 

 the instrument. 



For regular work with X rays it would probably be found 

 convenient to use a fine phosphor-bronze suspension, and 

 connect the needle through this permanently to the source of 

 potential. However, as I have shown, a change in 



the 



