The earliest machine which has any right to be called a dynamo was of this class. Barlow and Sturgeon had shown that a copper disk, placed between the poles of a magnet, rotates in the magnetic field when traversed by an electric current from its axis to its periphery, where there is a sliding contact. Faraday,in 1831, showed that by rotating a similar disk mechanically between the poles of a magnet, continuous currents were obtained. These he drew off by collecting springs of copper or lead, one of which toadied the axis whilst the other pressed against the amalgamated periphery. Here was demonstrated the production of a permanent (i.e. continuous) current of electricity by ordinary magnets. He went on to employ the principle of separate excitement of his field - magnets. Effects were obtained from electro - magnetic poles, resulting from the use of copper helices or spirals, either alone or with iron cores. The directions of the motions were precisely the same; but the action was much greater when the iron cores were used than without.
Such a machine as Faraday's is impracticable for several reasons: the peripheral friction is inadmissible on any but a small scale; and the disposition of the field - magnets necessarily evokes wasteful eddy - currents in the disk, which, even if slit radially, would not be an appropriate form of armature for such a limited magnetic field.
Another method of obtaining a con. tinuous cutting of the lines of force, is where a sliding conductor travels round the pole of a magnet. Faraday even generated continuous currents by rotating a magnet with a sliding connection at its centre, from which a conductor ran round outside, and made contact With the end - pivots which supported the magnet.
A similar arrangement was devised by Varley about 1862. He rotated an iron magnet in a vertical frame, having a mercurial connection at the centre. The current which flowed from both ends of the magnet toward the centre was made to return to the machine, and to pass through coils surrounding the poles of the rotating magnet; thus anticipating the self - exciting principle of later date. Varley also proposed to use an external electro - magnet to increase the action.
Quite recently, the same fundamental idea has been worked upon by Siemens and Halske, who have produced a so - called " unipolar" machine. In this remarkable dynamo are 2 copper cylinders, both slit longitudinally to obviate eddy - currents, each of which rotates round one pole of a U - shaped electromagnet. A second electro - magnet, placed between the rotating cylinders, has protruding pole - pieces of arching form, which embrace the - cylinders above and below. Each cylinder, therefore,, rotates between an internal and external pole of opposite polarity, and consequently cuts the lines of force continuously by sliding upon the internal pole. The currents from this machine are of very great strength, but of only a few volts of electromotive force. To keep down the resistance, many collecting brushes press on each end of each cylinder. This dynamo is actually at work for electro - plating.
In Voice's dynamo, a coil armature, wound upon an iron ring, is so placed that the iron ring is itself one pole of a magnet, a projecting pole - piece from the other pole being fixed near it, so that the coils fixed upon one pole glide round and cut the lines of force .proceeding from the other pole. Whether this machine will be a practical one remains to be seen.
The following rules and regulations are drawn up by a committee of the Society of Telegraph Engineers and Electricians for the reduction to a minimum, in the case of electric lighting, of those risks of fire which are inherent in every system of artificial illumination, and also for the guidance and instruction of those who have, or who contemplate having, electric lighting apparatus installed in their premises. The difficulties that beset the electrical engineer are chiefly internal and invisible, and they can only be effectually guarded against by " testing," or probing with electric currents. They depend chiefly on leakage, undue resistance in the conductor, and bad joints, which lead to waste of energy and the dangerous production of heat.' These defects can only be detected by measuring, by means of special apparatus, the currents that are, either ordinarily or for the purpose of testing, passed through the circuit. Should wires become perceptibly warmed by the ordinary current, it is an indication that they are too small for the work they have to do, and that they should be replaced by larger wires.
Bare or exposed conductors should always be within visual inspection, and as far out of reach as possible, since the accidental falling on to, or the thoughtless placing of other conducting. bodies. upon, such conductors, would lead to "short circuiting,'1 and the consequent sudden generation of heat due to an increased current in conductors not adapted to carry it with safety.
The necessity cannot be too strongly urged for guarding against the presence of moisture and the use of " earth " as part of the circuit. Moisture leads to loss of current and to the destruction of the conductor by electrolytic corrosion, and the injudicious use of " earth " as a part of the circuit tends to magnify every other source of difficulty and danger. The chief dangers of every new application of electricity arise from ignorance and inexperience on the part of those who supply and fit up the requisite plant. The greatest element of safety is therefore the employment of skilled and experienced electricians to supervise the work.
(1) The dynamo machine should be fixed in a dry place. (2) It should not be exposed to dust or flyings. (3) It should be kept perfectly clean and its bearings well oiled. (4) The insulation of its coils and conductors should be practically perfect. (5) All conductors in the dynamo room should be firmly supported, well insulated, conveniently arranged for inspection, and marked or numbered.
(6) The Wires
(6) Every switch or commutator used for turning the current on or off should be constructed so that when it is moved and left it cannot permit of a permanent arc or of heating. (7) Every part of the circuit should be so determined that the gauge of wire to be used is properly porportioned to the currents it will have to carry, and all junctions with a smaller conductor should be fitted with a suitable safety fuse or protector, so that no portion of the conductor should ever be allowed to attain a temperature exceeding 150° F. (65 1/2° C.). (8) Under ordinary circumstances, complete metallic circuits should be used; the employment of gas or water pipes as conductors for the purpose of completing the circuit should not in any case be.allowed. (9) Bare wires passing over the tops of houses should never be less than 7 ft. clear of any part of the roof, and all wires crossing thoroughfares should invariably be high enough to allow fire escapes to pass under them. (10) It is roost essential that joints should be electrically and mechanically perfect, and united by solder.
The form of joint recommended is shown in Fig. 84.
(11) The position of wires when underground should be clearly indicated, and they should be laid down so as to be easily inspected and repaired. (12) All wires used for indoor purposes should be efficiently insulated, either by being covered throughout with some insulating medium, or, if bare, by resting on insulated supports. (13) When these wires pass through roofs, floors, wails, or partitions, or where they cross or are liable to touch metallic masses, like iron girders or pipes, they should be thoroughly protected by suitable additional covering; and where they are liable to abrasion from any cause, or to the depredations of rats or mice, they should be efficiently encased in some hard material. (14) Where indoor wires are put out of sight, as beneath flooring, they should be thoroughly protected from mechanical injury, and their position should be indicated. N.B. - The value of frequently testing the apparatus and circuits cannot be too strongly urged. The escape of electricity cannot be detected.by the sense of smell, as can gas, but it can be detected by apparatus far more certain and delicate.
Leakage not only means waste, but in the presence of moisture it means destruction of the conductor and its insulating covering, by electric action.
(15) Arc lamps should always be guarded by proper lanterns to prevent danger from falling incandescent pieces of carbon, and from as - cettding sparks. Their globes should be protected with wire netting. (16) The lanterns, and all parts which are to be handled, should be insulated from the circuit.
(17) Where bare wire out of doors rests on insulating supports, it should be coated with insulating material, such as indiarubber tape or tube, for at least 2 ft. on each side of the support. (18) To secure persons from danger inside buildings, it is essential so to arrange and protect the conductors and fittings, that no one can be exposed to the shocks of alternating currents of a mean electromotive force exceeding 100 volts, or to continuous currents of 200 volts, (19) If the differ ence of potential within any house exceeds 200 volts, the house should be provided with a " switch," so arranged that the supply of electricity can be at once cut off.
With reference to par. (10), Bolas says that the best way to make an electrical joint is, first to thoroughly tin the wires, and then wipe them carefully while they are still hot; any chloride of zinc which may have been used being next removed by a damp cloth. The wires are then bound, and subsequently well grouted with solder, rosin only being used as a flux.
Killingworth Hedges, in a paper recently read before the British Association, alludes to some sources of danger not previously mentioned. Thus, in reference to the development of heat caused by an increased resistance, he recalls Matthiessen's experiment showing that the conducting power of " commercial" copper wire is only 13.6 as against 99.95 for pure copper: hence the wire used must be pure throughout. An absolute essential is a cut - out or fusible plug in the circuit, arranged to melt if the current is more than 10 to 15 per cent. in excess of the working strength.