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DYNAMO the inductor is mounted on an iron cylinder the almost universal adoption of copper as the material for If, however, 3 the armature1 inductors is now seen to lie in its high (Fig. 4), a sufficient margin being allowed for mechanical clearance between the inductor conductivity. Since the voltage of the dynamo is the second factor to and the poles, not only will the rewhich its output is proportional, the conditions which luctance of the magnetic circuit render the induced E.M.F. a maximum must evidently be be reduced, and the total flux and reproduced as far as possible in practice, if the best use is its density in the air-gap B,; be to be made of a given mass of iron and copper. The first thereby increased, but the path of problem, therefore, in the construction of the dynamo is the lines will become nearly radial, the disposition of the inductors and field in such a manner except at the “ fringe ” near the that the three directions of field, inductors, and movement edges of the pole-tips, and hence are at right angles to one another, and so that the relative the relative directions of the movemotion is continuous. Reciprocating motion, such as ment and lines will be continuously would be obtained by direct attachment of the inductors at right angles. The shape of the E.M.F. curve will then be as to the piston of a steam-engine, has been successfully employed only in the special case of an “ oscillator ”2 shown in Fig. 4—flat-topped, with rounded corners rapidly producing a small current very rapidly changing in sloping down to the zero line. With the addition of a number of similar inductors direction. Rotary motion is therefore universally adopted, and with this two distinct cases arise. Either (A) the arranged along the length of the armature core parallel to length of the inductor is parallel to the axis of rotation, or the first, the question arises of their connexion, in order to (B) it is at right angles to it. A single inductor cannot, add their E.M.F.’s. Since the E.M.F.’s generated by the however, be made to give more than a few volts, and inductors under the one pole-piece are opposite in direction hence it is necessary to use several inductors, and to add to those induced under the other up their E.M.F.’s by connecting them in series. Two pole-piece as viewed from one end methods of effecting this may be distinguished, and as of the armature (Fig. 5, where each may be applied both to case (A) and to case (B), four the dotted and crossed wires indicate E.M.F.’s directed towards types of armatures result therefrom. (A). If an inductor is rotated in a rectangular gap and away from the observer), two formed in a horse-shoe magnet by two pole-pieces having distinct methods are possible by plane faces opposed to each other, as in Fig. 3, not only is which two or more inductors can be joined in series. the density of the flux in the gap (1) The first, or winding, between the poles small, but the was invented by Dr Antonio Pacinotti of Florence 4 in I860,, direction of movement is not and was subsequently and independently reintroduced in always at right angles to the 18705 by the Belgian electrician, Zenobe Theophile Gramme, direction of the lines. Starting whence it is also frequently called the “ Gramme ” windfrom the position of the inductor ing. By this method the farther end of inductor 1 (Fig. when it is mid-way between the 5) is joined to the near end of inductor 2; this latter lies pole-tips, its rate of line-cutting next to it on the surface of the core or immediately above will increase to a maximum as it it, so that both are simultaneously under the same polepasses under the centre of a polepiece. For this connexion to be possible, the armature face, and will then decrease to core must be a hollow cylinder, supported from the shaft zero. As it crosses a line drawn on an open non-magnetic spider or symmetrically through the gaps hub, between the arms of which between the poles, the direction there is room for the internal wire of the E.M.F. along its length is completing the loop (Fig. 6). The reversed, since its motion relatively end of one complete loop or turn to the direction of the field is reembracing one side of the armature versed. If the ends of the inductor are electrically connected to two collecting, rings core thus forms the starting-point fixed on, but insulated from, the shaft, two stationary for another loop, and the process brushes b b can be pressed so as to make rubbing contact can be continued if required to with the rings, and an external circuit may be. con- form a coil of two or more turns. nected to the brushes, which then form the “ terminals ” In the ring armature the iron core of the machine; the alternating E.M.F. set up in the serves the double purpose of coninductor will cause an alternating current to flow through ducting the lines across from one the external circuit, and the simplest form of alternator pole to the other, and also of is obtained. If the field cut by the inductor is of shielding from the magnetic flux uniform density, and all the lines pass straight across the hollow interior through which the connecting wires from one pole to the other (both of which assumptions are pass. Any lines which leak across the central space are approximately correct), a curve connecting the instantaneous cut by the internal wires, and the direction of cutting values of the E.M.F., as ordinates, with time or degrees of is such that the E.M.F. caused thereby opposes the revolution, as abscissae (as shown at the foot of Fig. 3), E.M.F. due to the inductors proper .on the external will be sinusoidal when the speed of rotation is uniform. surface. If, however, the section of iron in the core 1

Faraday, Exp. Res., series ii. § 6, pars. 211, 213 ; series xxviii. § 34, par. 3152. ... - Invented by Nikola Tesla (Elec. Eng. vol. xiii. p. 83. Op. Brit Pat., Spec. Nos. 2801 and 2812, 1894). Several early inventors, e.g., Dal Negro in 1832 (Phil. Mag., third series, vol. i. p. 45), adopted reciprocating or oscillatory motion, and this was again tried by Edison in 1878.

3 The advantage to be obtained by making the poles closely embrace the armature core was first realized by Dr Werner von Siemens 0 Berlin in his “ shuttle-wound” armature (Brit. Pat., No. 210/, )■ 4 Nuovo Gimento, 19, p. 378, 1865. Elias of Amsterdam is by some credited with having used ring winding on a motor even earlier in 1860. __ 1071 5 Brit. Pat., No. 1668,1870. Comptes Rendus, 73, p. I/O, is/1-