THB LBYDEJf JABS.] 



UNDULATORY FORCES. ELECTRICITY. 



179 



conductor of an electrical machine to rest inside a glass 

 vessel partly filled with water, whilst his hand encom- 

 passed the outside, was astonished at receiving a power- 

 ful shock on attempting to remove the chain from the 

 vessel. The occurrence, or, at all events, its repetition, in 

 the town of Leyden, has given a name to the arrangement. 



A Leyden jar is easily made by coating any kind of 

 thin glass vessel with tinfoil, on both sides. They are 

 usually cylindrical glass jars, on the outside and inside 

 of which tinfoil is pasted to about two-thirds the height. 

 The reason the coating is not carried higher, is, that the 

 charge would rapidly discharge itself if the two coatings 

 were not separated to some distance. The bottom of 

 both, in and outside, should be also covered with foil. 

 The coatings are simply required as distributors of the 

 electricity, and, as such, only are they essential It is 

 well that the reader should bear this in mind, because 

 many persons imagine that the charge rests in or on 

 the tinfoil, instead of the glass. So far from this being 

 the case, a jar may be fitted with movable coatings, 

 which, after charging the jar, may be removed by means 

 of silk threads, and it will be found that the charge is 

 actually retained on the surface of the vessel. On the 

 top of the jar a wooden cover is placed, Fig. 19. 

 the object of which is to keep out dust, 

 <tc. Through the cover a wire is passed, 

 which communicates with the inside metal 

 lining by means of a chain. The' wire also 

 extends outwards, beyond the top of the 

 cover ; and at its extremity a brass knob is 

 placed, at which the charge is taken from 

 the conductor of the electrical machine. 

 19 represents the usual form of the 

 Leyden jar : a is the outside coating of 

 tinfoil ; b is the interval of glass between 

 the inside and outside coatings ; e is the 

 wire which extends into the jar ; and d is the brass knob 

 at which the charge is received. 



The annexed en- 

 graving (Fig. 20) 

 illustrates the mode 

 of charging the jar ; 

 a being the con- 

 ductor of the elec- 

 trical machine ; 6 

 the knob of the 

 jar ; c the wire 

 reaching into the 

 inside ; and d the 

 outside coating. 

 The jar may be 

 held in the hand 

 whilst being charged, and a wire should be attached to 

 its outside so as to maintain connexion with the c.u" h, 

 for an insulated jar cannot be charged. 



The length of time required to charge the jar, depends 

 on its size and the power of the electrical machine ; for 

 a small one say of twelve inches in the plate will not 

 fully charge a jar of large size. A seven-feet plate 

 machine, or the hydro-electric, will easily charge thirty 

 jars, two feet high and twelve inches in diameter, in one 

 minute or less ; and a single jar, of the same size, can 

 be charged several times per minute, by either the plate 

 or steam machine. 



If a jar be presented too long to the conductor of a 

 machine, it will discharge itself ; a brilliant spark, at- 

 tended with a loud, snapping noise, will take place ; and 

 it is more than likely that the discharge will force itself 

 through some portion of the glass, and so render the jar 

 useless for further experiments. 



The usual mode of discharging a jar is represented in 

 the following engraving. An instrument, called a dis- 

 charger, is employed, which consists of two brass knobs, 

 fixed to a bent wire, a a. The instrument is held by 

 the glass handle 6, and the wire is fixed on a joint e, 

 by means of which the two balls can be removed from 

 each other to any extent which may bo desired. The 

 jar being charged, the balls of the discharger are brought 

 into contact with the knob, d, of the jar and its outside 



coating, e, when a spark and a snap will be produced ; 

 Fig. Si. 



d 



and the electric equilibrium will be restored in a small 

 jar, and nearly so in one of large size. In using the 

 latter, a slight second discharge may occasionally be 

 observed, which arises from what is called the residual 

 charge. In very dry, frosty weather, we have found 

 that very large jars will actually charge themselves a 

 fact which we first became acquainted with by removiu-.; 

 one whilst holding the knob and the outside coating at 

 the same time. This, however, is a rare occurrence, be- 

 cause, unless a Leyden jar is kept very clean and dry, 

 dust and moisture soon settle on the glass surface, and 

 so prevent the electrical separation of the two coatings. 

 In frosty weather, and during a dry east wind, some 

 curious results are often obtained from Leyden jars ; and 

 it is a good rule to apply the discharging-rod to every 

 part of an insulated coating before setting the apparatus 

 aside, because the charge will sometimes remain for 

 days on its surface. Large jars have the necessary dis- 

 advantage of being made with thick glass, which prevents 

 their being readily charged. They, however, must be 

 used for large machines. It is usual to connect a num- 

 ber of either the large or small size together, when 

 powerful electrical effects are desired. Such an arrange- 

 ment is called a Leyden Battery ; and by its use some 

 most interesting natural phenomena may be readily 

 illustrated. One of these is represented in the following 

 engraving ; in which it will be observed, that all the in- 

 sides are connected together by means of rods which are 

 fastened to the knobs of the jars, and the outsides rest 

 in .1 box lined with tinfoil, by means of which they are 

 all, electrically speaking, joined together. It is advis- 

 able, however, to fix a wire so that it may pass round 

 each jar, and it should be extended to a gas or water- 

 pipe, so as to ensure the ready charging of the jars. In 

 Fig. 22, this is eflected by means of a wire attached to 



Fig. 32. 



the hook in the side of the box. A rod should extend 

 from the knobs of the jar to the conductor of the ma- 

 chine ; and thus the battery is to be charged. The mode 

 of discharging it will be noticed as we proceed with our 

 experiments. We should here state, that unless the 

 single jar, or each one in a Leyden battery, is kept per- 

 fectly free from dust and moisture, it is impossible to 

 charge them. They should be carefully wipeil with a 

 warm silk handkerchief before use, and heated to a little 

 beyond the temperature of the room in which the expe- 

 riments are to be tried; and it is also frequently 



