Mr Richardson, On an attempt to detect radiation, etc. 171 
joined to b by thin pressure tubing, and they were clamped inde- 
pendently so that by keeping b fixed and raising the clamp of A, 
the wire was lifted out of the mercury cup and a spark gap 
produced. The apparatus was made light-tight with black paint, 
except for a small space near C which was generally covered with 
black paper held in place by a rubber-band. This enabled the 
spark gap to be observed when desired, and the quality of the 
discharge to be ascertained. The tube i was placed vertically in 
the gap between the pole pieces of an electromagnet, which were 
close up against it. The width of the air gap was thus equal to 
