50 On a Condensing Collector for Frictional Electrical Machines. 



movable, or can be expanded and contracted by simple sliding. 

 The interior wire should not quite reach the end of the tube, 

 because this is apt to be imperfectly annealed and easily cracks. 

 Neither should the outer coating, if metallic, be tight and rigid; 

 for I found that a tight brass slider, at once carrying the tube 

 and forming the exterior coating, caused speedy rupture by 

 spontaneous discharge. After several failures, the arrangement 

 above described enabled the tube to bear a high charge without 

 damage. 



We may, however, with some advantage dismiss the conduc- 

 tor (which is commonly ill adapted to retain electricity at so 

 high a potential), and substitute for it a long open tube mounted 

 at one end with points and at the other with a brass knob, the 

 points and knob communicating through the tube by a fine 

 wire. I constructed a collector of this kind with a portion of 

 a barometer-tube of -^o inch diameter and ^ inch bore, 19 

 inches long, and coated near the middle with tinfoil for 1 inch. 

 This worked remarkably well, giving sparks 4 inches long ; but 

 a little additional length and a somewhat larger bore would be 

 an improvement. For a large machine a piece of steam-gauge 

 tubing would be suitable. The stem which carries the tube 

 may be metallic, or of wood with a wire passing along it. 



I observed that the full power of the machine was not yet ex- 

 hibited, and therefore made a wooden point-holder supported by 

 a glass stem. In the upper surface is an aperture in which, 

 with a cork, a Leyden tube can be fixed in an upright position. 

 A piece of wire twisted about the outer coating (which may be 

 of paper rubbed with plumbago) communicates with a metallic 

 knob. We may also fix the closed end of the tube in the aper- 

 ture and connect the interior wire with the metallic ball. The 

 free electricity on the covered surface of the tube is thus got rid 

 of to some extent. This apparatus enabled me to obtain fine 

 sparks in air between balls over 5^ inches apart. The electri- 

 city, however, was frequently discharged over the glass of the 

 cylinder ; and it seemed that the limit allowed by the conditions 

 of the insulation was nearly attained. I believe this result is 

 remarkable for so small a machine. 



The apparatus is so simple, compact, and effective that it 

 should certainly be associated with every frictional electrical 

 machine. Leyden jars are made use of in connexion with Holtz's 

 machine with wonderful success ; but the smallest I have seen 

 are altogether too large for ordinary machines, and, as I have 

 said, the pith of the matter is that the condenser must be charged 

 and discharged frequently at the highest potential the machine 

 can give. Of course it is desirable to varnish the tubes and 

 otherwise take the usual precautions. 



