can be insulated when required; and for the purpose of allowing them to be charged and discharged with precision; they are connected with what may be considered as two centres of action; the first of these consists of a brass ball, (a, figs. 3. 14. 16) which slides with friction on a metallic rod, (c d) so as to admit of its being adjusted to any required altitude; it it has a number of small holes, drilled in its circumference for receiving the points of the connecting rods of the jars The rod which sustains this ball, is either insulated on a separate foot, and connected with the conductor of the machine, as in fig. 16; or is otherwise inserted directly into it, as in fig. 14. The second centre, consists of a larger ball of metal, (b) attached to a firm foot, and placed on the same conducting base with the jars, so as to have a perfect connection with it. The wire (nb fig. 14) employed to transmit the explosion, terminates in this ball.

When it is required to give the first centre of action (a) fig. 3. a separate insulation, the insulating rod of glass, is screwed immediately into the lower ball; (b) and sustains the metallic rod above described, by the intervention of a ball of baked wood, (d) as seen in figs. 3 and 16; the opposite extremity of the rod terminates in a similar ball, (c) through the substance of which, the conducting communication with the machine passes when it is placed on a separate foot. All the metallic connections are covered with sealing wax, except at the points of junction; and the wood balls and different insulations are carefully varnished.

4. To transmit the explosion at any required period, two different methods were employed, according to the circumstances of the experiment-In the first, the discharge is effected by means of the instrument represented in fig. 4. There are two balls of brass (a) (b) placed one over the

* Rosin or brimstone answers very well for this purpose; the latter is easily moulded to any convenient form, by pouring it in a state of fusion into common drinking glasses.

other about three inches apart-The lower one, (b) is insulated and is connected with the positive side of the jar or battery; and the upper one, (a) with the negative side through the centres of action before described. (3) The ball (a fig. 4) connected with the negative centre, is attached to a rod of brass, (d) which falls freely through a smaller ball, (c) when allowed to do so. It is preserved at a given distance by a curved and finely pointed brass wire, (n c d) inserted into it; this wire being moveable about a centre (at n) can, by means of a glass handle, be readily disengaged; and thus the upper ball (a) is allowed to descend on the ball beneath it, by which the accumulation is always transmitted in a certain and invariable way, and without leaving any residuum in the battery.

5. In the second method, the charge is allowed to proceed until it can break through a known interval; to measure which, a discharging electrometer is employed, fig. 5 on the principle first proposed by Mr. Lane, and described in vol. 51 of the Transactions of the Royal Society-the interval between the balls (a and b) being adjusted by a screw and index, the motion of which last is shewn by a graduated circle; (cn) thus the interval may be regulated with very great precision: the connexion with the opposite sides of the jar are made through the two centres of action, as in the preceeding case.

This discharging electrometer is also occasionally used in its more ordinary and simple form, as represented in fig. 7; the only difference being that the insulating portion (ed) terminates in balls of varnished wood, one of which (c) sustains it, whilst the other (d) contains the tube, through which the graduated metallic slide passes: there is likewise an additional ball, (b) placed in the charging rod, from which the jar discharges itself.

G

6. For the purpose of investigating the operation of the attractive force of the accumulation, and the law of its action under different conditions, I availed myself of a delicate and accurate balance, in the following manner:

The beam (m n fig. 6) is sustained in the required position, between two vertical rods of glass; (c) (d) a covered wire indicated by the dotted line, (ef) passes through one of these and connects it with the negative coating. From one of the arms, (m) á hollow gilded ball of wood, (a) is suspended by a metallic thread; this ball is about two inches in diameter, and weighs about 160 grains. From the opposite arm, is suspended a light brass pan, (p) by means of silk lines in the usual way-in this pan, is placed as much additional weight as is requisite to balance the ball just mentioned; and to put the whole mass in a state of equilibrium. The attractive force of the accumulation, is caused to act directly on the suspended ball (a) by means of an insulated ball of brass, (b) of the same dimensions, which is fixed directly under it, and is connected with the positive coating; it is so placed, that it can be depressed from contact with the suspended ball, through given distances, by means of a cylindrical slide, (r) to which it is attached, and a socket (t); the slide (r) has a scale engraved on it, divided into twentieths of an inch, and is supported on a glass pillar, by means of a varnished ball of baked wood, in which the socket is fixed, and through which the connection with the positive coating passes.

It will be immediately perceived, that in this arrangement the attractive force acts directly between the balls, (a) (b) and it can therefore be measured under any given condition, by weights, placed in the pan, (p) suspended from the opposite arm of the beam. The pan is allowed to rest on a small circular support, (q) the elevation of which can be changed so as to accommodate it to the horizontal position of the beam, and check any oscillation: there is also a small stop, (8)

inserted into this stand, which projects over the pan, and prevents the further descent of the beam, after the equilibrium is destroyed; without which, the explosion would pass, and destroy the gilding of the ball.

7. Lastly, to measure the effect of a given accumulation, an electrometer was employed, represented by fig. 8: it is very simple in its construction, being little more than an air thermometer, having a metallic wire passed air tight through its bulb; it consists of a glass tube, (ab c) whose interior diameter is somewhat less than the tenth of an inch; one of its extremities is bent upwards and outwards for about two inches, and is united by welding to a glass cup, at (c) which is of a large diameter and contains some coloured spirit: the bulb (mn) is about three inches in diameter, and with its transverse wire above mentioned is screwed air tight upon this cup. The opposite leg of the tube is sustained by a graduated scale, (ab) fixed on a convenient base, (b) and the point at which the fluid rests is marked zero. When an electrical explosion is passed through the wire, the fluid will be observed to ascend along the scale. The method of fixing the wire is easy; two flanches of brass, (m n, fig. 9) with projecting screws and shoulders, are cemented in and over the holes drilled through the glass, the wire is passed directly through the bulb by means of corresponding holes in these flanches, and being gently put on the stretch, is secured by short metallic or wood pegs, by which it is slightly compressed and retained in its situation. When metallic pegs are used it is necessary to have a small longitudinal notch cut in them. Both the pegs and extremities of the wire project a little for the convenience of removal; and thus wires of various kinds, and of different diameters, may be easily substituted.* The whole is finally rendered air tight by means of small balls of brass, which are made flat at one extremity, and screwed on the projecting parts of the flanches, against a collar of leather as shewn in fig. 9.

* See Transactions of the Royal Society, for 1827, pt. i. p. 18.

The metal best suited for measuring the force of an explo sion in this way, is platinum; it is easily acted on, and not liable to oxidation. The wire may vary in diameter, between the 50th and 150th of an inch, according to the circumstances of the experiment. This method of estimating the effects of electrical explosions by their action on metals, will be found very convenient, and it is susceptible of much greater accuracy than the ordinary means by the fusion of metallic wire, which appears to be somewhat uncertain and precarious, when reduced to practice. *

9. In the detail of these inquiries, concerning the laws of electrical accumulations, our attention will be directed;1-to the quantity of matter accumulated. 2-to the intensity, or free action of the accumulation. 3 to the extent and disposition of the surface, on which the matter has been distributed. 4-to the effects of the explosion: the quantity of matter, the surface and the intensity, being varied according to certain conditions. †

10. The relative quantity of matter may be measured by the revolutions of the electrical machine, employed to produce the accumulation; for if one revolution produces one quantity, as it assuredly does, then two revolutions should produce double that quantity, and so on, supposing the machine to be in good and uniform action, during the given time; but the precise manner in which a charge thus accumulates, not having been clearly determined, it is necessary to demonstrate, by

* Cuthbertson's Practical Electricity, p. 186.

✦ The immediate cause of electrical phænomena, must be considered as an agent of a peculiar kind, and if material, differing essentially from the usual forms of matter. The hypothesis of materiality, has only been assumed that we may be the better enabled to connect the results of experiment, and reason on them;-the only legitimate end for which hypothesis is at all admissible; whether electrical agency be material, or no in the ordinary acceptation of the word, I do not presume to determine.