[M'LENNAN] ELECTRIC SCREENING IN VACUUM TUBES 95 
This occurred only at very low pressures when A acted as cathode 
and D as anode. The electrode C then slowly gained a very slight 
negative charge. Its amount, however, was not at all comparable with 
the charges obtained in the experiments previously described, and would 
be fully accounted for by a slight diffusion of negative ions through 
the opening in the ebonite plug. 
6. SUMMARY OF RESULTS. 
1. In passing a discharge through a vacuum tube, especially at low 
pressures, an electric conduction takes place along the surface of the 
glass which, unless specially guarded against, is apt to interfere with 
the study of true effects within the tube. 
2. In the use of Faraday cylinders in vacuum tubes good earth 
connections are very essential. 
3. Even very small openings in such cylinders exercise a disturbing 
influence. 
4. Cathode rays, when allowed to fall upon Faraday cylinders, do 
not impress electrical conductivity upon the gas within unless the cylin- 
ders are made of metal thin enough to permit the rays to pass through. 
The experiments were conducted in the Cavendish Laboratory and 
my thanks are due to Professor Thomson who proposed the investiga- 
tion and offered many valuable suggestions throughout its progress. 
