Description.

It consists of a glass cylinder a about 10 or 12 inches in diameter, and 15 to 20 inches in length, turning between two upright pillars of glass, B B, fixed to a stout mahogany base. Two smooth metal conductors, equal in length to the cylinder, and about one-third of its diameter, c c, are placed parallel to it upon two glass pillars, DD, which are cemented into two sliding pieces of wood E, by which their distance from the cylinder may be adjusted. One of the conductors has a cushion, , attached to it by a bent metallic spring, nearly as long as the cylinder, and about one inch or an inch and a half wide, to the upper part of which is sewed a flap of oil-silk, c, which should reach from the cushion over the upper surface of the glass cylinder, to within about an inch of a row of points attached to the side of the opposite conductor. The conductor to which the cushion is attached is called the negative conductor; the other collects the electricity of the glass, and is called the positive conductor. H, is an adjusting screw to regulate the pressure of the cushion upon the cylinder. The motion of the cylinder is in the direction of the silk flap, and may be communicated by a handle attached at 1, or by the multiplying wheel к. To put this electrical machine into good action, every part should be made perfectly clean Method of usand dry. The cushion is then anointed with amalgam, and applied by ing it. a gentle pressure to the cylinder. If positive electricity is required, it may be received from the conductor bearing the points, that supporting the cushion being uninsulated by a wire passing from it to the stand; if, on the contrary, negative electricity is required, it may be obtained from the insulated cushion cylinder, the other being uninsulated.

nonconductors.

102. Some bodies suffer electricity to pass through their substance, and are called conductors. Others only receive it upon the spot Conductors & touched, and are called nonconductors. The former do not, in general become electric by friction, and are called nonelectrics: the latter, on the contrary, are electrics, or acquire electricity by friction. They are also called insulators. The metals are all conductors; dry air, glass, sulphur, and resins, are nonconductors. Water, damp wood, spirit of wine, damp air, and some oils, are imperfect conductors.

103. Rarefied air admits of the passage of electricity; so does the Electricity Torricellian vacuum: hence if an electrified body be placed under passes through the receiver of the air-pump, it loses its electricity during exhaustion. So that the air, independent of its nonconducting power, appears to influence the retentive properties of Eodies in respect to electricity, by its pressure.

a vacuum.

No constant

104. There appears to be no constant relation between the state of relation be- bodies and their conducting powers: among sólids, metals are conducstate of bodies tors, but gums and resins are nonconductors; among liquids, strong aland their con- caline, acid, and saline solutions, are good conductors; pure water is

tween the

ducting pow

ers.

Some snb

an imperfect conductor, and oils are nonconductors; solid wax is almost a nonconductor, but when melted, a good one. Conducting powers belong to bodies in the most opposite states; thus the flame of alcohol, and ice, are equally good conductors. (BIOT, Traité de Physique, tom. ii., p. 213.) Glass is a nonconductor when cold, but conducts when red-hot; the diamond is a nonconductor, but pure and wellburned charcoal is among the best conductors.

105. There are many mineral substances which show signs of elecstances be- tricity when heated, as the tourmalin, topaz, diamond, boracite, &c.; by being heat- and in these bodies the different surfaces exhibit different electrical

ed.

tricities at op

states.

106. Whenever one part of a body, or system of bodies, is positive, Opposite elec- another part is invariably negative; and these opposite electrical states posite sides of are always such as exactly to neutralize each other. Thus, in the a body. common electrical machine, one conductor receives the electricity of of the glass cylinder, and the other that of the silk rubber, and the former conductor is positive and the latter negative; but if they be connected, all electrical phænomena cease.

107. The best electrical machine for experimental purposes is represented in the annexed sketch.

!

Description.

It consists of a glass cylinder a about 10 or 12 inches in diameter, and 15 to 20 inches in length, turning between two upright pillars of glass, B B, fixed to a stout mahogany base. Two smooth metal conductors, equal in length to the cylinder, and about one-third of its diameter, c c, are placed parallel to it upon two glass pillars, DD, which are cemented into two sliding pieces of wood E, by which their distance from the cylinder may be adjusted. One of the conductors has a cushion, , attached to it by a bent metallic spring, nearly as long as the cylinder, and about one inch or an inch and a half wide, to the upper part of which is sewed a flap of oil-silk, G, which should reach from the cushion over the upper surface of the glass cylinder, to within about an inch of a row of points attached to the side of the opposite conductor. The conductor to which the cushion is attached is called the negative conductor; the other collects the electricity of the glass, and is called the positive conductor. H, is an adjusting screw to regulate the pressure of the cushion upon the cylinder. The motion of the cylinder is in the direction of the silk flap, and may be communicated by a handle attached at 1, or by the multiplying wheel к. To put this electrical machine into good action, every part should be made perfectly clean Method of usand dry. The cushion is then anointed with amalgam, and applied by a gentle pressure to the cylinder. If positive electricity is required, it may be received from the conductor bearing the points, that supporting the cushion being uninsulated by a wire passing from it to the stand; if, on the contrary, negative electricity is required, it may be obtained from the insulated cushion cylinder, the other being uninsulated.

1

Amalgam for

108. The best amalgam is composed of one part of tin and two of zinc exciting elec- melted together, and mixed, while fluid, with six parts of hot mercury in an iron mortar. This mixture is triturated till it becomes a fine powder, which is then formed into a tenacious paste with hogs' lard.

aicity.

109. Another form of the electrical machine consists of a circular glass plate A, mounted upon an axis and rubbed by two pairs of cush

ions, as shewn at в в. The brass conductor c has its points opposed Description. to the plate, and is insulated by the glass stem D.-E E are double pieces of oil-silk passing from the cushions to near the points. The whole is supported by a stout mahogany frame, and motion is given to the plate by the winch F.

These electrical machines have considerable power; they are easily cleaned and excited, and are more portable than the cylinders; but as they cannot be conveniently insulated, the negative electrical power cannot be well exhibited, so that for the purposes of experimental research the former machines are preferable.

and in the more perfect vacuum of a good air-pump, it is of a purplish hue, and scarcely visible except in a very dark room.

112. If an insulated conductor be electrified, and an uninsulated conductor be opposed to it, there being between the two a thin stratum of air, glass, or other nonconductor, the uninsulated conductor, under such circumstances, acquires an opposite electrical state to that of the originally electrified insulated conductor. In this case, the uninsulat- Electricity by ed body is said to be electrified by induction and the induced electri-induction. city remains evident, until an explosion, spark, or discharge happens, when the opposite electricities annihilate each other. Induced electricity may thus be exhibited through a long series of insulated conductors, provided the last of the series be communicated with the earth.

Thus, in the following diagram, a, may represent the positive conductor of the electrical machine; B, C, and D, three insulated conductors, placed at a little distance from each other, D having a chain touching the ground; then the balls 1, being positive, will attract the balls Illustrations 2, which are rendered negative by induction. Under these circumstances, each of the conductors becomes polar, and the balls 3 are positive, while 4 are negative, 5 positive, 6 negative, &c.: the central points of the conductors, в с D, are neutral. When these opposite electrical states have arrived at a certain intensity, sparks pass between the different conductors, and the electrical phænomena cease.

113. The extent of such a polar arrangement may be greatly increased by pasting small spangles of tinfoil, upon a clean plate of glass, within a small distance of each other, each of which will then represent an insulated conductor; and the first spangle being held near the excited conductor of the machine, and the last in the hand, a series of brilliant sparks will pass between each, indicating the annihilation of the opposite electrical states. The spiral luminous tube, a, luminous words, flowers, &c., are arrangements of this kind.

114. Upon the principle of induction it is that the accumulation of électricity in the Leyden phial is effected. It consists of a thin glass jar, coated internally and externally with tinfoil to within a short distance of its mouth. When the inner surface is rendered positive by union with the conductor of the electrical machine, the exterior, being connected with the ground, becomes negative by induction. When the inner and outer surfaces are united by a conductor, all electrical accumulation is annihilated by a powerful spark, and the two opposite states are found to have been precisely equivalent.

Spiral lugatnous tube.

Leyden phial.