are arranged in two columns of equal length, which are separately insulated in a vertical position: the positive end of one column is placed lowest, and the negative end of the other, their upper extremities being connected by a wire, they may be considered as one continuous column. A small ball is situated between each extremity of the column and its insulating support; a brass ball is suspended by a thin thread of raw silk, so as to hang midway between the balls, and at a very small distance from them.

"For this purpose the balls are connected during the adjustment of the pendulum by a wire, that their attraction may not interfere with it; and when this wire is removed, the motion of the pendulum commences. The whole appararus is placed

upon a circular mahogany base, in which a groove is turned to receive the lower edge of a glass shade, with which the whole is covered."

Mr. Singer directs that, in order to preserve the power of the columns, the two ends should never be connected by a conducting substance for any length of time. It is therefore necessary, when laid by, that it should be placed upon two sticks of sealing-wax, and that the terminal balls be half an inch or so from the table.

If a column which appears to have lost its power be thus insulated for a few days, it will recover. There is another cause of deterioration, which is more fatal: this is too much moisture. The paper discs therefore should be made as hot as possible before they are put together; or even subjected to a continued but gentle heat for some time before they are inclosed in the glass tube, and, that being heated also, the plates may be inclosed without the presence of any appreciable moisture.

FIG. 196.

The size of the plates may be ths of an inch in diameter, or less. With a column of 20,000 alternations a Leyden jar may be charged, and minute sparks are visible when contact is made with the fine points of wire connecting the two extremities.

When the dry pile is attached to the electroscope of Hare by substituting the poles of two of De Luc's columns for the gilt disc (Fig. 193, p. 212), the instrument is made wonderfully delicate, so much so that Mr. Sturgeon describes an arrangement of this kind, the delicacy of which he states to be such that, the cap being of zinc, and of the size of a sixpence, the pendent leaf is caused to lean towards the negative pole by merely pressing a plate of copper, also the size of a sixpence, upon it, and when the copper is suddenly lifted up the leaf strikes. The different electrical states of the inside and outside of various articles of clothing were readily ascertained by this delicate electroscope. Bohnenberger has the credit of making the first of these instruments.

The gold leaf, being in equilibrium, and neither attracted or repelled, is instantly moved to one side or the other when the very smallest amount of electricity is evolved on the cap of the instrument.

From these various experiments with electroscopes it may be learned that friction under every circumstance, and even when disguised, as in the rapid evaporation of water from a hot surface, is an important source of electricity;

That there are two kinds or conditions of electricity which exactly neutralize each other, and they are always evolved together;

That pressure, or any modification of mechanical motion, such as fracture, rending, or tearing, all cause electrical quiescence to be disturbed;

That heat, as applied to various crystals, sets their particles in motion, and causes the evolution of electric force;

That chemical action is a source of electricity, of a tension similar, though not equal, to that of ordinary friction, as shown in De Luc's column.

A, the gold-leaf suspended between the two poles, B B, of the dry pile.

ELECTRICAL MACHINES.

In the year 1777, Tiberius Cavallo, a thoroughly practical and learned electrician, describes, in his "Complete Treatise on Electricity," the construction of the cylinder electrical machine of his day. It will not be found to differ materially from that made in 1868. He remarks

"The principal parts of the electric machine are the electric, the moving engine, the rubber, and the prime conductor, i.e., an insulated conductor which immediately receives the electricity from the excited electric."

The electric formerly used was made of different substances, as glass, resin, sulphur, sealing-wax, &c.; and in different forms, as cylinders, globes, spheroids, &c. (Fig. 198.)

The three glass globes are made to rotate and rub against three cushions. The conductor, a piece of metallic pipe or a gun-barrel, was suspended from the ceiling by silken cords, and connected with the globes by unravelled gold lace hanging down, the latter being used for the same purpose as the points now attached to all conductors of electrical machines.

"This diversity," continues Cavallo, speaking of the various shapes and nature of the electric used, "then obtained on two accounts: first, because it was not ascertained which substance or form would answer best; and, secondly, on account of producing a negative or positive electricity at the pleasure of the operator; for, before the electricity of the insulated rubber was discovered, sulphur, rough glass, or sealing-wax was generally used for the negative elec

tricity." The reader will perceive that Cavallo adopts the Franklinian theory. "At present smooth glass only is used; for, when the machine has an insulated rubber, the operator may produce positive or negative electricity at his pleasure, without changing the electric.

"In regard to the form of the glass, those commonly used at present are globes and cylinders.

"The cylinders are made with two necks; they are used to the greatest advantage without any axis (or rod passed through from neck to neck); and their common size is from 4 in. diameter and 8 in. long to 12 in. diameter and 2 ft. long, which are perhaps as large as the workmen can conveniently make them.

"The glass generally used is the best flint, though it is not yet absolutely determined which kind of metal is the best for electrical globes and cylinders. The thickness of the glass seems immaterial, but perhaps the thinnest is preferable.

"It has often happened that glass globes and cylinders in the act of whirling have burst in innumerable pieces with great violence, and with some danger to the bystanders. Those accidents are supposed to happen when the globes and cylinders, after being blown, are suddenly cooled.

"It will, therefore," prudently remarks Cavallo, "be necessary to enjoin the workmen to let them pass gradually from the heat of the glass-house to the atmospherical temperature."

The author prefers a single handle, instead of the multiplying gear, which is very apt to get out of order, and the cord to stretch or break when most

wanted. The various parts of the machine just described were gradually invented and applied by various clever electricians,-Otto Guericke, Hawkesbee, Abbé Nollet, Dr. Watson, Wilson, Nairne, Dr. Priestley; and many years elapsed before the machine attained anything like the perfection of that em

FIG. 199.-Cylinder Electrical Machine, used by Cavallo in 1777, Showing the glass cylinder A A, with a pulley attached to one neck, E, round which an endless cord passes to a large or multiplying wheel, c; the cushion E, and silk flap F: the cushion, placed on a glass pillar let into a piece of wood, moves backwards and forwards in a groove, G, and is sccured by a screw; before use, is covered with amalgam. The machine is clamped to the table at H. The prime conductor II, with collecting points K, is supported on glass legs, L L, let into a mahogany stand. The amalgam used by Cavallo consisted of two parts mercury and one of tinfoil, with a little powdered chalk, all rubbed up with grease.

ployed by Cavallo in his experiments. A more elegant and compact form is now given to the cylinder machine by Messrs. Elliott, of the Strand.

The most convenient form is undoubtedly the plate electrical machine. Of this Cavallo says

"Next to Dr. Priestley's machine, I shall describe another, which was invented by Dr. Igenhouz, and which, from its simplicity and conciseness, makes a fine contrast with the former.

"This machine consists of a circular glass plate, about 1 ft. in diameter, which is turned vertically by a winch fixed to the iron axis that passes through its middle; and it is rubbed with four cushions, each about 2 in. long, situated at the opposite ends of the vertical diameter."

Fig. 200 is a drawing of the large plate electrical machine in use at the Polytechnic. The plate glass is 7 ft. in diameter, and rather more than ths of an inch thick; it has two large rubbers, and, when these are well amalgamated, and the weather is propitious-at least dry-very long sparks of great intensity may be obtained; when the atmosphere is damp, in spite of the rapidity and power with which it is turned round by a four-horse power steam engine, it will hardly give a spark an inch in length.

The prime conductor is a large globe, about 3 ft. in diameter; and inserted into this is a large ring of wood, 4 ft. in diameter, and raised 6 ft. from the globe-being an arrangement first proposed in connection with the Austrian electrical machines exhibited in the Great Exhibition of 1862. The ring, no doubt, theoretically speaking, should act as a condenser, and assist, by induction, to increase the tension of the electricity; but whether it be due to the

height of the building in which the ring is placed, or from other causes, the effect produced did not appear to be increased by this addition to the apparatus. The power of an electrical machine is greatly influenced by the nature of the glass. There is a very fine-looking machine at the Polytechnic, constructed on the plan of the late Sir William Green Harris: the plate is 3 ft. in diameter; but, in consequence of the alkali of the plate glass, its power is very slight, and not half so good as that of many small cylindrical machines. The best amalgam for an electrical machine is made of I part of tin, 2 of zinc, and 6 of mercury. Melt the zinc and tin together, and, when approaching solidification, add the mercury, and stir till the whole is solid: if the latter is added when the alloy of zinc and tin is too hot, much of it may be dissipated in vapour; and the amalgam should be made under a chimney, to avoid the fumes of mercury. Sometimes the above are rapidly melted together, and then placed in a wooden box and shaken until quite cold. The shaking reduces the greater part to a fine powder, which may be sifted out and used