422 Natalie C. B. Allen and 2\ H. Laby: 



tested the Scliwarzschild law over a wide range of intensities, and 

 iound it to hold only for extreme values of the intensity. For a 

 certain range of intensity he found the Bunsen-Roscoe law to be 

 true. 



Experiments were made in order to test the truth of the assump- 

 tion that, when the product of energy of X-radiation and of time 

 of exposure Avas constant, the photographic effect is the same. The 

 energy of the X-radiation is, with the other conditions constant, 

 proportional to the current flowing through the tube. The two 

 halves of a plate were exposed separately; the current for the second 

 exposure was 1/100 of the current for the first exposure, the time 

 of exposure being correspondingly increased to 100 times its 

 original value. After development it was found that the densities 

 -of the two halves of the plate were widely different, showing that 

 the product of energy of radiation and of time of exposure does 

 not determine the density. 



In the small range of currents used in this* work, however, (from 

 .03 to .06 milliampere) the statement that density is jointly pro- 

 portional to the energy of radiation and to the time is approxi- 

 mately true. This conclusion is verified by measurements made by 

 Kroncke.'i- His results are shown below : — 



Current 



Time 



Density 



The energy of the cathode-rays is jointly proportional to the 

 current flowing through the tube and to the electrical pressure 

 applied to the tube. The fraction of this cathode-ray energy converted 

 into X-ray energy in the Coolidge tube is proportional to the square 

 of the velocity of the cathode-rays, i.e., to the first power of the 

 pressure. 2 Hence we define exposure in radiography as 



e'''ct/d^volt^ampere.sec.cln -'. 



where, e volts = pressure applied to tube. 



c amperes = current through tube. 



t seconds = time of exposure. 



d cms = distance from source (anticathode) to plate. 

 'Ihe average current was measured by an unshunted milliammeter 

 connected in series with the tube and induction coil. The cathode 

 of the tube (to which the milliammeter was connected) was earthed. 

 The time of exposure was measured by a stop-watch. 



1 Knincke. Arm. d. Phys., iv., ItS, 1914, p. 687. 



2 Rutherford and BarMes. Phil. Ma<f., xxx., 1915, \\ 361. 



1.0 



2.0 



3.0 



4.0 milliumperes. 



12.0 



6.0 



4.0 



3.0 minutes. 



1.09 - 



1.15 - 



1.16 



1.15 



