The Production of X Pays of different Penetrative Values. 223 



of the Huhmkorff coil, and thereby increasing the difference of the 

 electrical potential between the cathode and the anode portions of 

 the tube, as measured by the length of the alternative spark in air. . . 



Again, similar results are obtained without 

 alteration to the vacuum or to the power of the 

 Ruhmkorff coil by varying" the resistance of the 

 tube by means of a, magnetic field.* 



In this case the gradual strengthening of the 

 magnetic field produces a gradual decrease in the 

 resistance of the tube and of the difference of the 

 electrical potential between the cathode and 

 anode, and, at the same time, causes a gradual 

 diminution of the penetrative value of the X 

 rays. 



I have further found that it is possible to vary 

 the penetrative value of the X rays produced in 

 a focus tube by simply altering the distance 

 between the cathode and the anti-cathode. 



For this experiment the tube, fig. 1, was con- 

 structed, in which the anti-cathode B of alumi- 

 nium, faced with platinum, is connected to the 

 anode terminal by a sliding steel rod, so that it 

 could be moved along the axis of the tube, and 

 the distance between cathode C and centre of the 

 anti-cathode B varied from 1 to 3 inches, the 

 anti-cathode being always outside the focus of the 

 cathode. The tube was exhausted to a degree 

 that gave a maximum contrast between the bones 

 and flesh of the hand when the anti-cathode was 

 midway between its two extreme positions, and 

 was then sealed off, so that the vacuum was not 

 varied during subsequent experiments. Upon the 

 anti- cathode being approached to the cathode, the 

 X rays immediately became of a more penetrative 

 value, just as though the vacuum had been in- 

 creased, while, at the same time, the potential 

 difference, as measured by the alternative spark, 

 was found to have risen. Again, when the anti- 

 cathode was moved in the opposite direction and placed at a greater 

 distance from the cathode, the potential difference fell, and the 

 X rays became less penetrative and similar to those produced at a 

 lower vacuum. In this way, without varying the vacuum, the pene- 



* See "The Effects of a Strong Magnetic Field upon Electric Discharges in 

 Vacuo" ' Koy. Soc. Proc.,' vol. 60, p. 179, and note in ' Nature ' for July 9, 1896, 

 p. 239. 



