October, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



,6^ 



The Celluloid Electric 

 Machine. 



By Charles E. Benham. 



The value of celluloid as an electric has hardly been 

 fully realised, and the use of it has, perhaps, been pre- 

 vented from considerations of its inflammability. 

 There is no danger whatever on that score in employing 

 it in the manner about to be described. The electric 

 properties of celluloid are somewhat mysterious. It is 

 not a particularly good non-conductor, yet at the least 

 friction with silk or other suitable rubbers it is highly 

 electrified. Its electric potency increases, and often a 

 sample that is quite new will hardly give any electricity, 

 and yet after a few weeks it becomes powerfully excited 

 by friction, and Dr. J. N. Goldsmith, of the Xylonite 

 Works at Brantham, states that this increase of 

 efficiency goes on to a maximum, and then, after a 

 course of many years, gradually declines. It is not 

 nearly so sensitive to atmospheric conditions as glass or 



I he Celluloid Electric Machine. 



[The illustralion shows a machine with two i6-inch glass 

 plates, mounted about half an inch apart on one spindle. The 

 outer surfaces of the plates are varnished with celluloid, and 

 each is lightly pressed by a silU cushion seen on the left. The 

 inner surfaces of the discs have tinfoil sectors which are 

 earthed by a brush placed between the plates where they pass 

 the silk rubbers. The prime conductor is connected with a 

 small Leyden jar, a pointed wire projecting from it between 

 the discs to collect the charge from the sectors. The attach- 

 ment for the X-ray tube is also shown. One terminal of the 

 tube is connected with a small insulated brass ball, placed 

 about half an inch from the prime conductor. A wire is led 

 from the other terminal to the outer foil of the Leyden jar.] 



most other electrics, and the machine about to be de- 

 scribed will start readily under ordinary conditions with- 

 out any preliminary warming. The spark is not, of 

 course, so strong in wet weather, as is also the case 

 with the influence machine, but the falling off is 

 probably diK^ to the defective insulation of the parts of 

 the machine imdcr such circumstances rather than to 

 any difference in the action of the celluloid itself. 



In the .'Xpril number of " Knowledge " a simple gas 

 lighter made of celluloid was briefly described, and it 

 may be mentioned that such a device may be made 

 ec|U;illy well, or, perhaps, with advantage, by employing 

 a glass tube coated with celluloid instead of a tube 



composed of that material. Celluloid is readily soluble 

 in amyl acetate, and the varnish thus formed may be 

 applied to glass, and when dry there will be a very 

 tenacious film of celluloid on the surface which becomes 

 highly electrified under the influence of a silk rubber. 



Instead of coating a glass tube in this way, the sur- 

 face of a glass disc may be varnished with celluloid, 

 and a very eflicient plate machine of extremely simple 

 design may thus be constructed. 



It should be mentioned here that a curious mistake 

 is made in the construction of the old " plate machine," 

 which, of course, is not often seen nowadays except in 

 class-rooms as illustrative of electric principles. This 

 mistake consists in the use of two rubbers — one on each 

 side of the glass. If one rubber on one side of the glass 

 is used and in place of a rubber on the other side an 

 earthed brush grazes the glass, there will be quite as 

 much electricity produced as with the two rubbers; 

 while much less friction is involved. The brush in- 

 creases the capacity of the plate where the rubber is 

 applied, and a second rubber, instead of the brush, only 

 does the same thing less effectively. The side away 

 from the rubber and touched by the brush may advan- 

 tageously be furnished with tinfoil sectors. Each 

 sector passing the brush when it is under the influence 

 of the rubber has its capacity largely increased, and 

 when free from the brush, passes the collectors with 

 correspondingly reduced capacity, and, consequently, 

 yields its charge to them readily. This applies quite as 

 much to the old-fashioned plate machine as to the cellu- 

 loid machine, and it is strange that the arrangement 

 never seems to have been adopted. 



The celluloid machine is made upon the plan just de- 

 scribed, the rubber being of silk, and no amalgam be- 

 ing used. Japanese silk is particularly good for the 

 purpose. There is also no special advantage in making 

 the plate pass the rubber twice in the course of a revolu- 

 tion, as in tlie Ramsden plate machine. Winter's plan 

 of single collectors for each revolution will be found the 

 best. It is equally a question whether any advantage 

 is gained by making the collectors embrace both sides 

 of the glass. Experiments do not at all indicate that 

 there is any increase in the output thereby. 



All these considerations make it clear that for an 

 eflicient machine the construction will be of the simplest 

 kind with the accompanying advantage of a minimum 

 of leakage, which is sure to be introduced together with 

 any unnecessary complications of the apparatus and an 

 increase of the number of [Kirts. The revolving disc, 

 turned with winch and pulley, passes a silk rubber 

 pressing against the left side of the disc, and at the 

 same spot the sectors on the opposite surface are 

 successively earthed by a brush. Passing round, the 

 sectors deliver their charge to the collectors, a row of 

 points connected with a prime conductor and facing the 

 sectors on the right hand side of the disc. Care must 

 be taken that the rubber presses only lightly against the 

 glass, so as to avoid anv risk of breakage. The cellu- 

 loid varnish is applied to the back of the glass only, and 

 should not be brought up to the centre of the disc, for, 

 as already stated, it is not a very good insulator. All 

 the rest of the disc, except, of course, the surface of the 

 tinfoil sectors, should be coated with shellac varnish. 



A more powerful machine may be made by using two 

 such discs with the tinfoil sectors facing each other. 

 The two discs should be mounted on the spindle so 

 that they are about half an inch apart. A rubber 

 is placed against each di.sc on the outside surfaces 

 at the left, a double brush on a wire support being 

 fixed between the two discs so as to touch the sectors of 

 each simultaneously. The collector is simply a pointed 



