﻿116 Mr. E. W. B. Gill on the Variation with Distance 



terminals averaged about 10 volts. The coil used was capable 

 of producing an 8-inch spark. 



The rajs produced passed through the two circular slits 

 in the thick lead ends of the brass tube AB ; the slits were 

 4/5 mm. in diam., 11 cm. apart, and A was distant 10 cm. 

 from anode ; suitable screens were placed to prevent the 

 rays leaving the box in any other direction than through the 

 tube. A third lead disk C with a central hole of the same 

 size was placed midway down the tube, to prevent as far as 

 possible secondary rays from the ends and sides of tube 

 emerging from B. 



The end slits completely determined the beam of rays, and 

 by drawing the figure to scale, or by calculation, it will be 

 found that practically all points of the anode (which was 

 circular, 1*8 cm. in diameter, and at an angle of 45° to line 

 of slits) contributed radiation to the emergent beam, this 

 being desirable for large ionization ; and that at 100 cm. 

 from anode the breadth of the beam was roughly 7 cm. 

 On drawing the figure of rays it will be at once seen that 

 they do not form a cone, but that the arrangement consists 

 roughly of a cone of rays diverging from a point midway 

 between the end slits combined with a cylindrical bundle of 

 rays 4*5 mm. in diameter. The final result of the experiment 

 being what it was, this arrangement is as good as any other ; 

 but had the law been different from the inverse square, it 

 would have been necessary to devise other arrangements. 



The rays emerging from B passed between the plates of 

 a small parallel plate detector D. The plates of this were 

 2 cm. in diameter and 1 cm. apart, one being charged to 

 80 volts by small storage-cells, the other joined to an electro- 

 meter through an insulated key. 



The function of this small detector was really to see that 

 the bulb was emitting constant radiation. 



The ionization produced by the beam was measured by the 

 large parallel plate detector E, placed first in the position 

 shown in fig. 1, and then in that shown in fig. 2. 



This detector had as one plate a thin wire gauze, while the 

 other plate was of brass placed parallel to the wires and 

 T5 cm. from them. The diameter of the brass plate, which 

 was circular, was 9 cm., thus allowing a margin of 2 cm. 

 over the beam, which, as was mentioned, spread out to 7 cm. 

 at the longer distance. To create a uniform field for the ions 

 produced, a guard-ring 1J cm. wide enclosed this plate. 



The wire gauze was charged to a potential of 80 volts, the 

 brass plate connected to an electrometer, and the deflexion 

 read due to the Rontgen ravs being on for 1 minute. This 



■^H 



