Nov., 1904.] 



KNOWLEDGE .<c SCIENTIFIC NEWS. 



269 



An Electric Influence 

 Experiment. 



Interesting Illustration of the Prin = 

 ciple of the Wimshurst Machine. 



Bv Chakles E. Ben ham. 



The following simple experiments will bo found ontortainin.!,' 

 in themselves, and at the same time they will help to elucidate 

 the principle of the remarkable accumulation of electricity 

 which occurs in the well-known Wimshurst intluonce machine. 



Take four pieces of s; hiss — half-plate negative glass (6' by 4|) 

 will do very well — and mount a 3-inch circle of tinfoil on each, 

 a little above the centre, as shown in fig. i. Coat the glass 

 well on both sides with shellac varnish, which, by the way, 

 should always be filtered through a few thicknesses of fine 

 muslin to ensure a good smooth surface. 



Fix two of the glass plates, which we will call .\ and B, edge 

 to edge horizontally, with the tinfoil downwards, holding them 

 in a \ice. or pushing their ends into a grooved block of wood 

 as shown in fig. 2. 



Lay the two other plates, C and D, with tinfoil upwards, on 

 the fi.xed plates, C on A and D on B. The tinfoil not being 

 central, the glass will project beyond the edge of A and H, 

 thus enabling C and U to be handled and moved as required 

 in the experiment. 



Touch C and D with the finger and they will receive from A 

 and B respectively an infinitesimal charge of electricity, prob- 

 ably quite imperceptible, even with a delicate electroscope. 

 Remove the finger, and then without displacing C turn I) over 

 on to it, so that the two tinfoils of the upper plates are in con- 

 tact. The surface of C D will now have the combined charges 

 of C and D. Touch A, and it will receive by induction a 

 charge equivalent to this combined charge — still an impercep- 

 tible one. Lay the pair on B and touch B, which will likewise 

 receive a similar double charge. Open out the two plates and 

 repeat the whole process. It will be seen on consideration 

 that the second time C and D will receive practically double 

 the charge they first took up. At the third operation they will 

 again double their last charge, and so on. It is not really 

 quite double, but taking the accumulation as being practically 

 in that ratio, it means that in ten operations the original charge 

 will have increased more than a thousand times. At any 

 rate, before the tenth time sparks will probably be observed 

 at each touching of the tinfoils, and will be found to increase 

 every time until the limit of the capacity of the surfaces is 

 reached. 



To those who are familiar with the principle of the " doubler," 

 invented by Bennet more than a hundred years ago. there is 

 nothing surprising about this method of producing electricity, 

 but to the tyro, or to persons unacquainted with electric 

 phenomena, this creation of a powerful and increasing series 

 of sparks without any manifest original source cannot fail to 

 seem astonishing. 



It is evident that while electricity is produced by this simple 

 process — and with large plates is produced in considerable 

 quantities — it would be mechanically inconvenient to construct 

 an apparatus to go through these intricacies of touching and 

 transposing glass plates, but a further experiment will show 

 how the most troublesome part of the process may be dis- 



pensed with, and will at the same time illustrate in a striking 

 manner the very principle of the accumulative power of the 

 Wimsliurst machine. 



For this second experiment only three of tlie tinfoil discs 

 are required — A, C, and D — and it will be more convenient if 

 C and D are on one plate. A being fixed as before, lay the 

 larger plate upon it with the tinfoil C (uppermost) over A. 

 Charge C by touching as before. Then slide the upper plate 

 so that D is over A. and, by touching, charge that loo. C anil 

 I) now have between them double the charge of A, but of 

 course a charge of opposite sign. C and D cannot be brought 

 into contact as before, but if the plate is moved so that A is 

 midway between them (as in fig. j), A will be under the 

 inlluence of both, and will, when touched, receive a charge by 

 induction, and a repetition of the process will quickly result in 

 accumulation, so that all three discs will soon be strongly 

 charged. On separating them and touching each in turn, 

 strong sparks will be given off by each. 



The application of this principle in the Wimshurst machine 

 is at once obvious, though it has never perhaps been illus- 

 trated in this way before. I'"ach sector of the Wimshurst, as 

 it passes under the brush, is earthed while under the influence 

 of more than one sector of the other di.sc, for it is within the 

 field of at least two or three of them. The sector, when 

 earthed by the brush, is, in fact, in the position of A in fig. j, 

 while the adjacent sectors of the other disc are etpiivalenl to 

 C .and D in the above experiment. Kach of the four brushes 

 of the neutralising rods places a sector at a similar .advantage 

 for increasing its ch.arge, and hence the ready .accunmlation of 

 the Wimshurst machine. 



.•\fter this it also becomes evident why a certain number of 

 sectors are advisable for the Wimshurst machine. It will, 

 indeed, act without any sectors at all, but only while the 

 brushes are new .and very large, their ends that touch the glass 

 taking the place of the tinfoil. But in practice, the inventor 

 of the Wimshurst machine recommended th.it there should 

 never be less than a certain number nf sectors, the minimum 

 depending on the size of the plate. The reason for this 

 minimum would appear to be, in the light of the experiments 

 described above, that with widely separated sectors the 

 carriers would not be within the field of more than one at a 

 time, in which case the multiplication of the charge would not 

 take place. On the other hand, sectors can be too numerous. 

 This is not only because of the leakage involved, but also 

 because the carrier (represented by A in our experiment) 

 would then be under the inductive influence, not only of the 

 sectors of the other disc, but also of those to right and left of 

 it on its own disc, which sectors are charged with electricity of 

 sign similar to its own, and tend therefore to neutralise the 

 inductive effect of those on the other disc, which are of 

 opposite sign. 



Gl\jttonous AnimoLls. 



By R. LVDEKKER. 



Maw kinds of carnivorous animals, such as the larger 

 members of the cat tribe, are in the habit of 

 periodically eating very heavy meals, and then abstain- 

 ing altogether from food for several days, until the 

 pangs of hunger once more reassert themselves. 

 Pythons and certain other snakes come under the 

 same category, as does also the common or medicinal 

 leech. Such animals clearly are not to be classed as 

 gluttonous — they might almost as well be included 

 among fasting animals — they merely take in large sup- 

 plies of food at long intervals, and, on the average, do 

 not appear to devour more than a normal amount of 

 nutriment. Vultures, on the other hand, although 

 they likewise require a period of abstention from food 

 of some length after each gorge, do appear to con- 

 sume very much more than an average quantity of 

 food, and, therefore, strictly speaking, come within 

 the scope of the present article. 



