312 



THE POPULAR EDUCATOR. 



two coatings of a charged pane, we shall find that there is a 

 small amount of positive electricity on one side un-neutralised 

 by induction, and therefore free. The amount of this depends 

 on the thickness of the glass. 



If then we hold the finger to this side, taking care that the 

 other coating is insulated, we shall obtain a small spark from it. 

 As soon as this has passed, a similar amount of negative elec- 

 tricity will be set free at the other side, and we can then draw a 

 spark from that ; in this way, we may continue drawing oft the 

 charge, drop by drop, as it were, till it is all gone. If a Leyden 

 jar be placed on an insulating stand, we may discharge it in a 

 similar way. 



Two interesting experiments may be tried as illustrations of 

 this one with the pane, the other with the jar. Make two feet 

 of gutta-percha, or some insulating material, so that the pane 

 may stand vertically. Having charged it, take a piece of wire 

 and place a pith-ball on each end ; then bend it almost round, 

 so that when it rests on the upper edge of tho pane the balls 

 may each be about an inch from the coating. The side on 

 which the excess exists will first attract tho ball nearest to it, 

 and thus part with its excess of electricity ; the other ball will 

 then be attracted by 

 the other side, and 

 in this way the wire 

 will rock backwards 

 and forwards till the 

 charge is dissipated. 

 This is called the 

 electric pendulum. 



To show the same 

 thing with a Leyden 

 jar, we must fix a 

 bell on the wire just 

 below the ball. A 

 metal support, A. 

 (Fig. 16), carrying a 

 similar bell at the 

 same height, must 

 be fixed on a stand, 

 and connected with 

 the outer coating of 

 the jar by a piece of 

 wire or tinfoil, B. 

 The upper part of 

 this support has a 

 bent wire fixed to it, 

 from which a small 

 ball is suspended 

 by a thread of silk. 

 If now the jar be 

 charged, this ball 



will be alternately attracted and repelled by the bells, and 

 thus will continue to ring them till the jar is discharged. 



If a jar be charged, and allowed to stand, the electricity in it 

 will be slowly dissipated, chiefly owing to the moisture in the 

 air, which acts as an imperfect conductor. This may be partly 

 obviated by coating the surface of the glass with shellac 

 varnish, and thus hindering the deposit of damp which usually 

 forms on it. When, however, it is desired to preserve a charge 

 for any length of time, the construction of the jar is' slightly 

 altered. The rod is not fixed to the cap, but passes through a 

 glass tube fixed in it to the bottom ; and when the jar is 

 charged, it may be inverted, and the rod allowed to fall out. 

 When the charge is required, the rod must be carefully dropped 

 in again. In doing this, however, it is advisable to take the pre- 

 caution of placing it first upon some non-conducting substance. 



Theoretically, there is no limit to the size of the jar that may 

 be employed. In practice, however, many inconveniences attach 

 to tho use of those which are very large. The tension of the 

 electricity frequently becomes so great in them, that if there be 

 a flaw or thin place in the glass it will pierce it ; and if it be 

 made thick, to guard against this danger, the induction is con- 

 siderably weakened. The plan therefore adopted is to employ a 

 number of small jars, and connect them together so as to form 

 a battery. They are usually placed in a tray lined with metal, 

 so as to connect their exterior coatings, and a wire is brought 

 from this to one of the handles ; the knobs are also connected 

 together by wires passing through them. (Fig. 17.) 



Various batteries of great size and power have thus .been 

 made at different times : for all ordinary purposes, however, 

 from four to nine jars, holding about five or six pints each, will 

 be amply sufficient. 



It is important to be able to estimate with seme degree of 

 accuracy the intensity of the charge in a jar, and different 

 means of attaining this end have been devised. The simplest 

 is by means of the quadrant electrometer A (Fig. 17), which ia 

 shown on one of the knobs of the battery. It consists of a 

 thick brass rod, surmounted by a knob, and bearing on one side 

 a semi-circular graduated scale, usually made of bone or ivory. 

 At the centre of this is suspended a thin wood rod, carrying a 

 pith-ball on its further end. As the charge increases, this ball 

 becomes more and more repelled, and the angle shown on the 

 graduated arc increases, thereby giving a rough indication of 

 the intensity of the charge. It must be remembered, however, 

 that it is the intensity, and not the quantity, of the electricity 

 that is indicated. If the same amount of electricity be distri- 

 buted over jars or batteries having double the amount of coated 

 surface, the electrometer will only show one-half the intensity. 

 Still, the instrument is very useful, especially if it is made with 



the rod somewhat 



-., - ,,,,ur ^Mninr . smaller at the lower 



Fig. 16. 



fit into the prime con- 

 ductor or any other 

 piece of apparatus. 

 A better way of 

 regulating the inten- 

 sity of a shock is by 

 means of Lane's dis- 

 charging electrome- 

 ter, which is repre- 

 sented in Fig. 18. 

 It depends for its 

 action on the fact 

 that the distance 

 through which a 

 charge will dart be- 

 tween the balls, or, as 

 it is called, the strik- 

 ing distance, is, for 

 small charges, directs 

 ly proportional to 

 the intensity of the 

 charge ; that is, if 

 double the amount of 

 electricity bo present 

 on the same surface 

 the striking distance 

 will be twice as great. 



This distance varies, too, inversely as the amount of surface over 

 which tho charge is distributed. Thus, if one jar has twice the 

 surface of another, and the same amount of electricity be passed 

 into each, the striking distance of the first will be only one-half 

 that of the second. These results will easily be understood if we 

 recollect that it is always intensity, and not quantity, that is 

 shown ; just as a thermometer shows the intensity and not the 

 quantity of heat, for there clearly is a larger quantity in a gallon 

 of water at the temperature of the body than in a cupful at tho 

 boiling-point, and yet the thermometer shows a much higher 

 temperature if immersed in the latter. 



The electrometer consists of a metal tube, A, which fits on to 

 the rod of a jar or battery, and carries at one side a curved rod 

 of glass, B. At the end of this is fixed a second tube, C, with a 

 brass wire, terminated at one end in a ball, r>, and at the other 

 in a ring, passing through it. When this is attached to a jar, the 

 ball D may be brought within any desired distance of the knob, 

 and a chain is then attached to the ring and connected with the 

 outside. When the jar is sufficiently charged, the electricity 

 darts between the balls and discharges it. If it be desired to 

 pass this shock through the body, or through any substance, the 

 two sides of it are connected by brass chain or wire with the ring 

 and outer coating respectively, and the shock passes as before. 



There are one or two other forms of discharging electrometer 

 which are sometimes used, and which depend on the attraction 

 of the knob of the jar for a balanced metal rod. Wo need 

 not, however, stop to explain them here. 



