KLECTRfCITY. 



829 



tion of electricity arising from induction, is by the 

 employment of coated glass, that is, of a plate of 

 glass, on each side of which is pasted a sheet or coat- 

 ing of tinfoil. Care must be taken to leave a suffi- 

 cient margin of glass uncovered by the metal, for 

 preventing the transfer of electricity from one coat 

 ing to the other, round the edge of the glass ; and 

 all sharp angles, or ragged edges in the coatings, 

 should be avoided, as they have a great tendency to 

 dissipate the charge. The form of coated glass best 

 adapted to experiments is that of a cylindric jar ; 

 this is coated, within and without, nearly to the top. 

 The cover consists of baked wood, and is inserted 

 with sealing-wax, to exclude -moisture and dust. A 

 metallic rod, rising two or three inches above the 

 jar, and terminated at the top in a brass knob, is 

 made to descend through the cover till it touches the 

 interior coating. The name of the Leyden phial, or 

 jar, is applied to this instrument. 



In Fig. 4 we have given a view of the Leyden jar, 

 as it is usually constructed ; in which figure, / re- 

 presents the knob at the top of the wire which passes 

 through the wax-coated cover, and reaches the 

 bottom of the jar e e. In this figure, the jar is re- 

 presented as standing upon an insulated stool ; that 

 is, a stool, the top of which is made of baked, f. e. 

 well dried wood, and the feet of glass, which prevents 

 any passage of the electric fluid between the jar and 

 the earth. A chain d, is represented as connecting 

 the outside coating of the jar with the pith ball 

 electrometer baa, the stalk, c, of which is formed of 

 glass. By this arrangement, the relative quantities 

 of electricity in the jar may be determined by the 

 degree of divergence of the balls a a. The Leyden jar 

 is used in the following manner : the outer coating 

 being made to communicate with the ground, by 

 holding it in the hand, the knob of the jar is pre- 

 sented to the prime conductor when the machine is 

 in motion ; a succession of sparks will pass between 

 "them, while, at the same time, nearly an equal quan- 

 tity of electricity will be passing out from the exte- 

 rior coating, througli the body of the person who 

 holds it, to the ground. The jar, on being removed, 

 is said to be charged ; and if a communication is 

 made between the two coatings, by a metallic wire, 

 extending from the external one to the knob, the 

 electric fluid which was accumulated in the positive 

 coating, rushes, with a sudden and violent impetus, 

 along the conductor, and passes into the negative 

 coating ; thus at once restoring an almost complete 

 equilibrium. 



The conductor used for this purpose is called a 

 discharging rod, a very convenient form of which is 

 shown in fig. 5. It consists of a glass handle A, 

 carrying two bent wires terminated in brass knobs 

 B, C. The knobs may be placed at different dis- 

 tances from each other ; as the wires on which they 

 are fixed are movable round the joint, D, the in- 

 strument resembles a pair of callipers. In order to 

 use this discharging rod, the one knob is to be ap- 

 plied to the knob of the jar, and the other to the 

 outside coating ; the discharge instantly takes place, 

 and the electric equilibrium is restored. Another 

 discharging apparatus is shewn in fig. 9. It is called 

 Cuthbertson's discharging electrometer, and is the 

 best of those instruments that have been invented 

 for measuring the intensity of the electric charge, 

 by the weight which the electric force may be able 

 to overcome. Into a wooden frame a a, two glass 

 pillars are fixed, the one b, carrying the metallic ball, 

 c, and the other the ball, g. A brass rod df, is made 

 to pass through the ball c, is terminated by the two 

 metallic balls, d and/. This rod rests upon a knife- 

 edged centre within the ball C, and is capable of 

 moving vertically, on that centre being delicately 



balanced. Into the top of the ball c, another wire is 

 fixed, which carries the ball e directly above the ball 

 /. The ball g is made to communicate with the 

 outside coating of the jar, by means of the chain i, 

 and the ball c is made to communicate with the in- 

 side coating by means of (he chain A. On the por- 

 tion cf of the rod af, there is a slider which may 

 be moved to or from the centre C, as may be re- 

 quired. When the chain h and i are connected with 

 the two sides of the jar, as stated above, and the in- 

 strument in the position represented in the figure, it 

 is clear that no discharge can take place. Before the 

 iar can be discharged, the ball /, must be brought 

 into contact with the ball g, which is effected by 

 moving the slider along the arm c /, towards the 

 ball. The slider is so adjusted, that it indicates, by 

 its position on the arm, which is graduated, the 

 number of grains requisite to weigh down the ball/. 

 By the adjustment of the slider, the charge may be 

 compelled to continue to increase, until the force 

 exerted be equal to any required number of grains. 

 From the experiments of Mr Singer, it would appear 

 that this instrument is a tolerably exact measure of 

 electric intensity ; from which experiments it may 

 be inferred, that it is the same thing whether the 

 electric charge be doubled in intensity, or the coated 

 surface be doubled in extent. 



The sudden transfer of a large quantity of accu- 

 mulated electricity is a real explosion ; and it gives 

 rise to a vivid flash of light, corresponding in inten- 

 sity to the magnitude of the charge. The effect of 

 its transmission is much greater than that of the 

 simple charge of the prime conductor of the ma- 

 chine ; and it imparts a sensation, when passing 

 through any part of the body, of a peculiar kind, 

 which is called the electric shock. In the construc- 

 tion of the Leyden jar, the thickness of the glass is 

 an important consideration. The thinner the glass, 

 the greater will be the power of taking a charge ; 

 but the power of retaining it will be less, on account 

 of the diminished resistance which the glass will 

 oppose to the electricity through it. If the charge 

 be higher than what the jar will bear, the glass will 

 be broken by the violence with which the electric 

 fluid forces a passage through its substance. An- 

 other limit to the charge which a jar is capable of 

 retaining, arises from the liability of the electricity 

 to pass from one coating to the other, round the 

 edges of the glass. The deposition of moisture, 

 also, on the glass, will occasion a spontaneous 

 discharge, since it forms a chain of conducting par, 

 tides, in the very line which the electricity has a 

 tendency to take. Hence, in order to preserve the 

 uncoated part of the glass in as dry a state as possi- 

 ble, it is usually covered with a layer of sealing 

 wax, or some other resinous varnish. By uniting 

 together a sufficient number of jars, we are able 

 to accumulate an enormous quantity of electricity : 

 for this purpose, all the interior coatings of the jars, 

 must be made to communicate by metallic rods, and 

 a similar union must be established among the ex 

 terior coatings. When thus arranged, the whole 

 series may be charged, as if they formed but one jar ; 

 and the whole of the accumulated electricity may be 

 transferred from one system of coatings to the other, 

 by a general and simultaneous discharge. Such a 

 combination of jars is called an electrical battery. 

 In Fig. 11 we have given a view of an electrical 

 battery, consisting of twelve jars contained within 

 a case a a a a, of baked wood. The arrangement 

 of the wires and knobs, b b b b, will be easily under- 

 stood by an inspection of the figure. For the pur- 

 pose of making the direct communication between 

 the inner and the outer coating of a jar or battery, 

 by which a discharge is effected, the instrument cal- 



