SLIDING CONDENSERS. 455 



panes that is to say, condensers made of a glass plate with tin- 

 foil on each side, and having definite ratios to each other. He 

 had observed that the influence of the edges prevented the capacities 

 from being proportional to the surface ; and he found that, to allow 

 for this effect, it was sufficient to add to the real surface of the 

 tinfoil a circular band 1-5 mm. in breadth for glass 5 mm. thick, 

 and 2*25 mm. for glass 17 mm. thick. 



1045. SLIDING CONDENSERS. It is often useful to have capaci- 

 ties which can continuously vary. This is what is obtained with 

 a plate condenser provided with a guard ring, and in which the 

 disc, which moves parallel to itself, may be displaced in the di- 

 rection of the perpendicular by a micrometric screw. The same 

 result is arrived at more conveniently with condensers in which 

 one of the armatures moves parallel to itself, so as to vary the 

 extent of the two surfaces without altering the distance which 

 separates them. 



Consider in particular the system of conductors discussed in 98, 

 consisting of two cylindrical envelopes A and B (Fig. 214) with a 



C ! 



Fig. 214. 



common axis and a cylinder C having the same axis as A and B 

 and moving along this axis.* The envelope B being put to earth, 

 the cylinders A and C are connected with each other and insulated. 

 Let C be the capacity of the system formed by the union of A 

 and C when the system is in a given position which serves as mark, 

 a the capacity of unit length of the condenser CB for the mean 

 region where the distribution of the densities is uniform. If the 

 cylinder C be made to slide towards the right by a length x, 

 the capacity of the system AC becomes C Q + ax; it varies then 

 proportionally to the quantity x. This would also be the case 

 for the envelope B, if it were alone insulated, and the system AC 

 communicated with the ground. 



* The idea of this condenser is due to Sir W. THOMSON. See also GIBSON 

 and BARCLAY, Transactions of the Roy. Soc., 1871, p. 573. 



