846 3Ir. T. Thome BaJcer [April 22, 



In practical language the condition for compensation is that the 

 principal cell should have great sensitiveness and a small inertia con- 

 stant, the compensation cell low sensitiveness and a high inertia con- 

 stant, the product of sensitiveness and inertia constant being the 

 same in the case of both cells. 



The physical properties of selenium are of such importance that 

 I feel I may be allowed to digress for a few moments to show one 

 way in which they may be utilised to solve a problem that has long 

 occupied many investigators, viz., the satisfactory measurement of 

 the beam of heterogeneous rays from an X-ray tube. Whenever a 

 new tube is used in radiographic work, a different voltage, or different 

 interrupter or coil, the time of exposure for the photos^raphic plate 

 has to be determined anew. The strength of the tube under any 

 conditions can, however, be determined by means of a simple piece 

 of apparatus which I have conslructed, the working of which I shall 

 now be able to show you. 



If the X-rays fall on a fluorescent screen of Imrium platino-cyanide, 

 the screen absorbs them and emits yellowish-green visible rays ; this 

 transformed energy is capable of affecting a very sensitive selenium 

 cell when placed in contact w4th the screen, the resistance becoming 

 less the greater the fluorescence. You will see here a selenium cell 

 of approximately 895,000 ohms resistance, over which is placed a 

 small fluorescent screen of the same size ; the cell is put in series 

 with a battery of 100 volts and a milli-ampere metre, the divisions 

 of which may be made to correspond to some arbitrary scale, or to 

 the time necessary for the exposure of a given make of photographic 

 plate. 



The dividing of the dial depends on two things : first, the charac- 

 teristic curve of the selenium cell connecting its resistance with the 

 strength of illumination, the linear distance of the source from tlie 

 cell being in this case the most convenient to employ. 



Secondly, this characteristic curve must be modified to meet the 

 case of illumination by the rays from the anti-cathode, which do not 

 necessarily diminish in their power to make the screen fluoresce as the 

 square of the distance from it. You will see on the screen the charac- 

 teristic curve of a selected selenium cell for feeble illumination, 

 the maximum being of about the same wave-length as that of the 

 fluorescence, showing the relation between resistance and distance 

 separating the source of illumination and the cell, and also the 

 modified curve showing a similar relation betw^een resistance and 

 distance between anti-cathode and cell, with the screen in contact. 

 The portion of the first curve most nearly asymptotic is best to 

 employ for the work, and from the second curve the dial scale of 

 the metre can be easily calibrated. If, now, I vary the height of the 

 X-ray tube from the measuring apparatus, you will see that the metre 

 needle is deflected less as the distance between tube and cell is in- 

 creased. The actual instrument is provided with a scale divided 



