CONDUCTOR
, in Electricity, a term first introduced in this science by Dr. Desaguliers, and used to denote those substances which are capable of receiving and transmitting electricity; in opposition to electrics, in which the matter or virtue of electricity may be excited and accumulated, or retained. The former are also called non-electrics, and the latter non-conductors. And all bodies are ranked under one or other of these two classes, though none of them are persect electrics, nor perfect conductors, so as wholly to retain, or freely and without resistance to transmit the electric sluid.
To the class of conductors, belong all metals and semi-metals, ores, and all fluids (except air and oils), together with the substances containing them, the effluvia of flaming bodies, ice (unless very hard frozen), and snow, most saline and stony substances, charcoals, of which the best are those that have been exposed to the greatest heat, smoke, and the vapour of hot water.
It seems probable that the electric fluid passes through the substance, and not merely over the surfaces of metallic conductors; because, if a wire of any kind of metal be covered with some electric substance, as resin, | sealing-wax, &c, and a jar be discharged through it, the charge will be conducted as well as without the electric coating.
It has also been alleged, that electricity will pervade a vacuum, and be transmitted through it almost as freely as through the substance of the best conductor: but Mr. Walsh found, that the electrie spark or shock would no more pass through a perfect vacuum, than through a stick of solid glass. In other instances however, when the vacuum has been made with all possible eare, the experiment has not succeeded.
It may also be observed, that many of the forementioned substances are capable of being electrified, and that their conducting power may be destroyed and recovered by different processes: for example, green wood is a conductor; but baked, it becomes a non-conductor: again its conducting power is restored by charring it; and lastly it is destroyed by reducing this to ashes.
Again, many electric substances, as glass, resin, air, &c, become conductors by being made very hot: however, air heated by glass must be excepted.
See, on this subject, Priestley's Hist. of Electricity, vol. 1; Franklin's Letters &c, pa. 96 and 262 edit. 1769; Cavallo's Complete Treat. of Electr. chap. 2; Henley's Exper. and Obser. in Electr. also Philos. Trans. vol. 67 pa. 122; and elsewhere in the different volumes of the Transactions.
Prime Conductor, is an insulated conductor, so connected with the electrical machine, as to receive the electricity immediately from the excited electric.
Mr. Grey first employed metallic conductors in this way, in 1734; and these were several pieces of metal suspended on silken strings, which he charged with electricity. Mr. Du Fay fastened to the end of an iron bar, which he used as his prime conductor, a bundle of linen threads, to which he applied the excited tube: but these were afterwards changed for small wires suspended from a common gun-barrel, or other metallic rod.
In the present advanced state of the science, this part of the electrical apparatus has been considerably improved. The prime conductor is made of hollow brass, and usually of a cylindrical form. Care should be taken, that it be perfectly smooth and round, without points and sharp edges. The ends of the conductor are spherical; and it is necessary, that the part most remote from the electric should be made round and much larger than the rest, the better to prevent the electric matter from escaping, which it always endeavours most to do at the greatest distance from the electric: and the other end should be furnished with several pointed wires or needles, either suspended from, or fixed to an open metallic ring, and pointing to the globe or cylinder, or plate, to collect the fire. It is best supported by pillars of solid glass, covered with sealing-wax or good varnish. Prime conductors of a large size are usually made of paste-board, covered with tin-foil or gilt paper; these being useful for throwing off a longer and denser spark than those of a smaller size: they should terminate in a smaller knob or obtuse edge, at which the sparks should be solicited. Mr. Nairne prepared a conductor 6 feet in length, and 1 foot in diameter, from which he drew electrical sparks at the distance of 16, 17, or 18 inches; and Dr. Van Marum still far exceeded this, with a conductor of 8 inches diameter, and upwards of 20 feet long, formed of different pieces, and applied to the large electrical machine in Teyler's Museum at Harlem, the most powerful machine of the kind ever yet constructed. But the size of the conductor is always limited by that of the electric, there being a maximum which the size of the former should not exceed; for it may be so large, that the dissipation of the electricity from its surface may be greater than that which the electric is capable of supplying.
Dr. Priestley recommends a prime conductor of polished copper, in the form of a pear, supported by a pillar and a firm basis of baked wood: this receives its fire by a long arched wire of soft brass, which may be easily bent, and raised or lowered to the globe: it is terminated by an open ring, in which some sharp-pointed wires are hung. In the body of this conductor are holes for the insertion of metalline rods. This, he says, collects the fire perfectly well, and retains it equally everywhere. Philos. Trans. vol. 64, art. 7. Hist. Elect. vol. 2, § 2.
Mr. Henly has contrived a new kind of prime conductor, which, from its use, is called the luminous conductor. It consists of a glass tube 18 inches long, and 2 inches diameter. The tube is furnished at both ends with brass caps and ferules about 2 inches long, cemented and made air-tight, and terminated by brass balls. In one of these caps is drilled a small hole, which is covered by a strong valve, and serves for exhausting the tube of its air. Within the tube at each end there is a knobbed wire, projecting to the distance of 2 inches and a half from the brass caps. To one of the balls is annexed a fine-pointed wire for receiving and collecting the electricity, and to the other a wire with a knob or ball for discharging it. The conductor, thus prepared, is supported on pillars of sealing-wax or glass. Beside the common purposes of a prime conductor to an electrical machine, this apparatus serves to exhibit and ascertain the direction of the electric matter in its passage through it. See a figure of this conductor in the Philos. Trans. with a description of experiments, &c. with it, vol. 64, pa. 403.
Conductors of Lightning, are pointed metallic rods fixed to the upper parts of buildings, to secure them from strokes of lightning. These were invented and proposed by Dr. Franklin for this purpose, soon after the identity of electricity and lightning was ascertained; and they exhibit a very important and useful application of modern discoveries in this science. This ingenious philosopher, having found that pointed bodies are better fitted for receiving and throwing off the electric fire, than such as are terminated by blunt ends or flat surfaces, and that metals are the readiest and best conductors, soon discovered that lightning and electricity resembled each other in this and other distinguishing properties: he therefore recommended a pointed metalline rod, to be raised some feet above the highest part of a building, and to be continued down into the ground, or the nearest water. The lightning, should it ever come within a certain distance of this rod or wire, would be attracted by it, and pass through it preferably to any other part of the building, and be conveyed into the earth or water, and there dissipated, without doing any damage to the building. Many facts have occur- | red to evince the utility of this simple and seemingly trifling apparatus. And yet some electricians, of whom Mr. Wilson was the chief, have objected to the pointed termination of this conductor; preferring rather a blunt end: because, they pretend, a point invites the electricity from the clouds, and attracts it at a greater distance than a blunt conductor. Philos. Trans. vol. 54, pa. 234; vol. 63, pa. 49; and vol. 68, pa. 232.
This subject has indeed been very accurately examined and discussed; and pointed conductors are almost universally, and for the best reasons, recommended as the most proper and eligible. A sharp-pointed conductor, as it attracts the electric fire of a cloud at a greater distance than the other, draws it off gradually: and by conveying it away gently, and in a continued stream, prevents an accumulation and a stroke; whereas a conductor with a blunt termination receives the whole discharge of a cloud at once, and is much more likely to be exploded, whenever a cloud comes within a striking distance. To this may be added experience; for buildings guarded by either natural or artificial conductors terminating in a point, have very seldom been struck by lightning; but others, having flat or blunt terminations, have often been struck and damaged by it.
The best conductor for this purpose, is a rod of iron, or rather of copper, as being a better conductor of electricity, and less liable to rust, about 3 quarters of an inch thick, which is either to be fastened to the walls of a building by wooden cramps, or supported by wooden posts, at the distance of a foot or two from the wall; though less may do: the upper end of it should terminate in a pyramidal form, with a sharp point and edges; and, when made of iron, gilt or painted near the top, or else pointed with copper; and be elevated 5 or 6 feet above the highest part of the building, or chimneys, to which it may be fastened. The lower end should be driven 5 or 6 feet into the ground, and directed away from the foundations of the building, or continued till it communicates with the nearest water: and if this part be made of lead, it will be less apt to decay. When the conductor is formed of different pieces of metal, care should be taken that they are well joined: and it is farther recommended, that a communication be made from the conductor by plates of lead, 8 or 10 inches broad, with the lead on the ridges and gutters, and with the pipes that carry down the rain water, which should be continued to the bottom of the building, and be made to communicate either with water or moist earth, or with the main pipe which serves the house with water. If the building be large, two, three, or more conductors should be applied to different parts of it, in proportion to its extent. Philos. Trans. vol. 64, pa. 403.
Chains have been used as conductors for preserving ships; but as the electric matter does not pass readily through the links of it, copper wires, a little thicker than a goose quill, have been preferred, and are now generally used. They should reach 2 or 3 feet above the highest mast, and be continued down in any convenient direction, so as always to touch the sea water. Philos. Trans. vol. 52, pa. 633. See also Franklin's Letters &c 1769, pa. 65, 124, 479, &c; and Cavallo's Electr. chap. 9.
For the Construction and management of Electrical Kites, and Cenductors or Machines for drawing electricity from the clouds, see Priestley's Hist. of Electr. vol. 2, pa. 103 edit. 1775.
