May, 1913. 



KNOWLEDGE. 



165 



quantities of electricity. An insulated strip of gold 

 leaf is suspended between plates connected with the 

 terminals of a dry pile. The plates attract it equally, 

 and, therefore, it remains at rest, but the most 

 infinitesimal charge of electricity suffices to upset 

 this equilibrium and to make the gold leaf sway to 

 right or left according to the positive or negative 

 nature of the charge applied. This form of electro- 

 scope is of extreme sensitiveness, and has the 

 advantage of indicating the sign, positive or negative, 

 of the electricity present. 



While contact is the source of electrification the 

 intensity of effect from a single contact is so 

 infinitesimal that for practical purposes it is 

 absolutely necessary to increase it. It has just been 

 shown how this can be done by connecting in series 

 a large number of contact-making plates, but this is 

 not the only way in which an increased difference of 

 potential can be brought about. 



A second' means is by applying the principle of 

 nduction. When a charged body is brought near 

 an insulated conductor without touching it, and the 

 insulated conductor is momentarily earthed, it will 

 be found to have acquired an induced charge nearly 

 equal in amount to that of the inductor, though of 

 opposite sign. And this happens without robbing 

 the inductor of any of its electricity. In fact, by 

 repeating the process any number of induced charges 

 can be obtained without diminishing the electricity 

 of the source. This obviously affords the clue to a 

 way in which by a succession of induction processes 

 an accumulated charge of appreciable magnitude can 

 be obtained from even such an infinitesimal source 

 as that of a pair of metal plates in contact. Adding 

 together the induced charges they mount up to an 

 unlimited extent. 



But this process of repeated additions is necess- 

 arily slow. Much more rapid is the increase if 

 instead of merely adding them they can be multiplied. 

 Two induced charges, when combined, are practically 

 double the inductor's charge. Two induced charges 

 derived from this double charge will, of course, give a 

 fourfold charge. Two from this will give an 

 eightfold charge, and so on. The rapidity of 

 accumulation on this doubling principle is astound- 

 ing. Double a charge and its products in this 

 compound interest way twenty-four times, and it 

 increases more than eight million-fold. 



A very simple and pretty experiment will illustrate 

 this. Fix upright from the centre of a board a 

 disc of metal supported by a narrow insulating strip 

 of varnished glass. Provide two strips of similar 

 varnished glass a little wider than the disc's 

 diameter and in height not quite as tall as the top 

 of the disc, and let them have discs of tinfoil 

 attached near the upper part. The lower end of 

 each is glued to a block of wood, so that they can 

 be pushed along the board up to the inductor, one 

 on each side, the tinfoil being on the outer side of 

 the glass, i.e., away from the inductor. The 

 arrangement is shown in Figure 151. 



The inductor, presumably by its mere contact 



with the air, has an infinitesimal charge of electricity, 

 and this, solely by multiplied induced charges, can 

 be made to give out in a few minutes sparks half an 

 inch long. The effect is almost miraculous to the 

 uninitiated, as the electricity seems to come from 

 nothing. It really comes, not from nothing, but 

 from an imperceptible and infinitesimal beginning. 



The modus operandi is as follows : — Push one of 

 the mounted strips close up to the inductor. Earth 

 its sector by touching it with the finger. It has 

 received an infinitesimal charge by induction, 

 equivalent to that of the inductor. Slide it away 

 and do the same with the second glass strip. Now 

 bring both up to the inductor and touch the top of 

 its disc. It will, of course, receive by induction from 

 the combined influence of the two strips a double 

 charge. Repeat the cycle and it will gain a fourfold 

 charge. Repeat the process twenty-four times and 

 the original charge will have increased some eight 

 million times, and by this time at each earthing a 

 growing spark will be heard and seen, and its 

 intensity will go on accumulating until it is limited 

 by the capacity of the three conducting discs. 



This simple experiment illustrates very graphically 

 the underlying principle of practically every form of 

 electric influence machine, such as the Voss, the 

 Wimshurst, and Clarke's gas lighter. All these 

 machines merely provide for the rapid performance 

 of a cycle of inductions so arranged as to produce an 

 accumulation on the compound interest principle. 

 The original source of the electricity is always the 

 infinitesimal contact charge, generally that derived 

 from the mere contact of air with the tinfoil sectors 

 of the machine. 



Lord Kelvin applied the same principle to a 

 Water-dropping apparatus in which two streams of 

 water drops pass through the middle of cylindrical 

 metal inductors, each drop carrying an induced 

 charge as it breaks away from the stream under the 

 influence of the cylinder. The drops give up their 

 charges to metal cups which by means of cross wires 

 communicate the growing charges to the inductors. 

 Starting with no appreciable trace of electricity the 

 accumulation rapidly multiplies and causes the 

 startling phenomenon of flashing sparks between 

 contiguous parts of the apparatus, and the accumula- 

 tion is only limited by the fact that the streams of 

 drops become so highly charged that they soon repel 

 each other with such force that they scatter instead 

 of falling into their respective receivers. 



The same effect may be produced with two falling 

 streams of sand or metal filings. In every such case 

 the initial charge is derived from contact, and its 

 manifestation is due to the doubling principle 

 already explained. 



In a very rough and general way it is possible to 

 estimate the voltage of the original infinitesimal 

 charge of a doubler. Reverting, for example, to 

 our preliminary experiment with the strips of glass, 

 it will generally be found that about twenty-four 

 doublings yield a half-inch spark. It may be taken 

 that every tenth of an inch represents roughly a 



