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DYNAMO 587 possible uniformly upon the circuit, and as the winding must the armature is rotated, the small amount of flux left in the be more or less concentrated into coils, it resolves itself into the magnet is cut by the inductors, and a very small current begins to round the closed circuit; this increases the flux, which in Af. netic necessit y °f surfaces introducing as long as possible be- flow tween any which are air-paths at different magnetic turn further increases the E.M.F. and current, until, finally, the e ' potentials. No iron should be brought near the machine cumulative effect stops through the increasing saturation of the which does not form part of the magnetic circuit proper, and iron cores. Fig. 33, illustrating the series machine, shows the especially no iron should be brought near the poles, between which winding of the exciting coils to be composed of a few turns of the difference of magnetic potential practically reaches its maxi- thick wire. Since the current is undivided throughout the whole mum value. In default of a machine of the same size or similar circuit, the resistance of both the armature and field - magnet type on which to experiment, the probable direction of the leakage winding must be low as compared with that of the external circuit, flux must be assumed from the drawing, and the air surrounding if the useful power available at the terminals of the machine is to the machine must be mapped out into areas, between which the form a large percentage of the total electrical power—in other permeances are calculated as closely as possible by means of such words, if the efficiency is to be high. Fig. 34 shows the third approximate formulae as those devised by Professor Forbes.1 In the earliest “ magneto-electric ” machines permanent steel magnets, either simple or compound, were employed, and for many years these were retained in certain 0a a erna, rs offield‘ ^ to > some of which are still in use for arc magnet. lighting in lighthouses. But since the field they furnish is very weak, a great advance was made when they were replaced by soft-iron electromagnets, which could be made to yield a much more intense flux. As early as 1831 Faraday2 experimented with electromethod, in which the winding of the field-magnets is a shunt or magnets, and after 1850 fine-wire circuit of many turns applied to the terminals of 4the machine ; in this case the resistance of the shunt must be high as they gradually superseded compared with that of the external circuit, in order that only a the permanent magnet. small proportion of the total energy may be absorbed in the field. When the total ampereSince the whole of the armature current passes round the fieldturns required to excite magnet of the series machine, any alteration in the resistance of the external circuit will affect the excitation and also the voltage. the electromagnet have A curve connecting together corresponding values of external been determined, it recurrent and terminal voltage for a given speed of rotation is known mains to decide how the as the external-characteristic of the machine ; in its main features excitation shall be obtained; and, according to the method it has the same appearance as a curve of magnetic flux, but when adopted, continuous-current machines may be divided into the current exceeds a certain amount, it begins to bend downwards and the voltage decreases. The reason for this will be found in four well-defined classes. the armature reaction at large loads, which gradually produces a The simplest method, and that which was first used, is separate more and more powerful demagnetizing effect, as the brushes are excitation from some other source of direct current, which may shifted forwards to avoid sparking; eventually the back amperebe either a primary or a secondary battery or another dynamo turns overpower any addition to the field that would otherwise be (Fig. 32). But since the armature yields a continuous current, it due to the increased current flowing round the magnet. The was early suggested (by Brett in 1848 and Sinsteden in 1851) that “external characteristic” for a shunt machine has an entirely this current might be utilized to increase the flux; combinations different shape. The field-magnet circuit being connected in of permanent and electromagnets were therefore next employed, parallel with the external circuit, the exciting current, if the acting either on the main armature or on separate armatures, until in applied voltage remains the same, is in no way affected by altera1867 Dr Werner von Siemens and Sir C. Wheatstone almost simul- tions in the resistance of the latter. As, however, an increase in taneously discovered that the dynamo could be made self-exciting the external current causes a greater through the residual 3 magnetism retained in the soft-iron cores loss of volts in the armature and of the electromagnet. The former proposed to take the whole of a greater armature reaction, the terthe current round the magnet coils which were in series with the minal voltage, which is also the exarmature and external circuit, while the latter proposed to utilize citing voltage, is highest at no load only a portion derived by a shunt from the main circuit ; we thus and then diminishes. The fall is arrive at the second and third classes, namely, series and shunt at first gradual, but after a certain machines. The starting of the process of excitation in either case critical value of the armature curis the same ; when the brushes are touching the commutator and rent is reached, the machine is rapidly demagnetized and loses its 1 Soc. Tel. Eng. vol. xv. p. 555, 1886. Cp. also Ives, “ Magnetic voltage entirely. Leakage in Dynamos and Motors,” Elec. Rev., Jan. 22 and 29, 1892, The last method of excitation, and2 Puffer, Elec. Rev., April 15, 1892. namely, compound-winding (Fig. Exp. Res., series i. § 4, par. 111. In 1845 Wheatstone and Cooke 35), is a combination of the two prepatented the use of “ voltaic” magnets in place of permanent magnets ceding, 6and was first used by S. A. b (No. Yarley and by Brush. If a machine 3 In10,655). Pat. No. 3006, 1863, Wilde described the use of a small is in the first instance shunt-wound, Fig. 35. magneto-machine as exciter of a larger dynamo with electromagnets ; and a certain number of series-turns cp. also his paper read before the Royal Society, April 1866. In are added, the latter, since they carry the external current, can be 1866 C. and S. A. Varley provisionally protected a machine having made to counteract the effect which the increased external current electromagnets only, excited initially by the application of a direct have in lowering the voltage of the simple shunt machine. current for a short time (No. 3394, 1866) ; this appeared again in the would The ampere-turns of the series winding must be such that they not complete Patent No. 1755, 1867; but meantime the same device had only balance the increase of the demagnetizing back ampere-turns on been patented by Sir C. W. Siemens, Jan. 31, 1867, No. 261. Dr 4 Werner von Siemens’s paper was submitted to the Berlin Academy Sir William Siemens, Phil. Trans., part iii., 1880, “On the of Sciences, Jan. 1867 {Pogg. Ann., Feb. 1867), and his results were Dynamo-Electric Current, and on Certain Means to Improve its Steadicommunicated to the Royal Society in London by Sir William ness,” and Brit. Pat., No. 4534, 1879. 5 Siemens in a paper “ On the Conversion of Dynamical into Electrical In Brit. Pat., No. 4905,1876, he describes a machine with its fieldForce without the Aid of Permanent Magnetism,” which was read on magnet wound with two coils of different resistance ; and Brush (Brit. Feb. 14, 1867. On the same day Sir C. Wheatstone’s paper “ On the Pat., No. 2003, 1878) employed a shunt circuit to maintain the Augmentation of the Power of a Magnet by the Reaction thereon of magnetism of a series-wound machine. But the full advantages of Currents induced by the Magnet itself” was read before the Royal compound-winding were not realized until the patent of Crompton and Society. Kapp (No. 4810, 1882).