t 49 ] 



VII. On a simple Condensing Collector for Frictional Electrical 

 Machines. By Samuel Roberts, Esq.* 



THE prime conductor of a frictional electrical machine is 

 one armature of a condenser, the air being the dielectric, 

 and the more or less conducting objects in the neighbourhood 

 constituting the second armature. This obvious consideration 

 suggests the form of a collector suitable for developing the full 

 length of spark which the machine is capable of giving. Winter's 

 ring aims at an enlargement of the conductor with as little dis- 

 sipation as possible, and requires accurate and expensive work- 

 manship. The exterior armature remains irregular and variable 

 in the extreme ; and we depend on the extent of the intervening 

 dielectric for rendering the effect of these irregularities small. 

 If we control the form of the exterior armature as well as that 

 of the interior one, we really make an artificial condenser or 

 Leyden battery, though the dielectric may still be air. I revert 

 then to the old form of apparatus, in which, for medical pur- 

 poses, the prime conductor was superseded by a Leyden jar; 

 but as my object is not to accumulate a large quantity of elec- 

 tricity, but to produce long and frequent sparks, to obtain, in 

 fact, the ordinary phenomena of a prime conductor armed with 

 a ring and charged to a high potential, my Leyden jar is of 

 minute dimensions ; and in this consists what I conceive to be 

 the novelty. The result of my experiments was so satisfactory 

 that I venture to communicate it. The apparatus is very effec- 

 tive, of insignificant cost, and may be applied to any machine at 

 once. 



For a small machine with a cylinder 6 inches in diameter and 

 8 inches long, I take a glass tube -^ w inch in diameter and 13 

 inches long, hermetically sealed at one end. A fine copper 

 wire, gauge 25, is inserted ; and a piece of paper 2 \ inches broad 

 is wrapped four or five times round the closed end. The open 

 end with a little wire projecting is to be applied to the prime 

 conductor ; and the outer coating must be in good electric con- 

 nexion with a metallic ball by means of a similar wire twisted 

 once round the paper. I obtained in this way brilliant zigzag 

 sparks 4 \ inches in clear length. 



The charge of the tube at a given potential depends on the 

 interior surface of the tube, and in part on the exterior surface, 

 as well as on the wire and coating. Hence a tube of -^ inch 

 diameter and \ inch bore will give a spark of considerable 

 volume, which may occasion a disagreeable shock. We can, 

 however, modify the effect indefinitely ; and this is a great ad- 

 vantage of the arrangement. The outer coating may be made 

 * Communicated by the Author. 



Phil Mag. S. 4. Vol. 4/. No. 309. Jan. 1874. E 



