Electro-Dynamic Rotation By Means Of Alternating Currents

Thursday, October 18, 1888
Archived Page: 

sos gnéusfries, iso. May, isss. this case the line 0 lt representing the resultant tield revolves round the centre 0, the direction and magnitude of the resultant Held changing at every instcnt. If, however, there is no la 1 between the two currents, that is to say, if they pass tlirough zero simultaneously, the direction of the resultant field OR remains stationary, and the point R performs harmonic oscillation over the line lt U and its prolongation with O ascontro. This may be considered as ii special case of the ellipse where the latter has been flattened down to u. met. xcmatical line. Another special case occurs when the ellipse becomes n circle, and this must evidently take place when the lag between the two currents is !)0”, when the coils are at right angles to each other, and when the component lields are equally strong. A metallic body suspended in thc contre of the two fields, and free to revolve, must evidently have e tendency to participate in the rotation of the resultant field. '1`he application of this very simple principle willbe clear from Fig. 2, in which the two coils are indicated hy the rectangles A and B, the connections being made by the wires 1 l‘ and 22‘ respectively. When alternating currents are sent through these coils a resultant field is produced, which revolves round the line OO. In the centre between the coils is suspended a small hollow cylinder of copper, and as the lines of the resultant iicld cut the metal Foucault currents ere induced, end by the interaction of these induced currents and the revolving held the cylinder is set in rotation. The action is exactly the reverse of that occurring in the \vel.l known experiment in which it copper disc is rapidly revolved below a compass needle, with the result that the compass needle is deflected on account of the induced currents in the copper disc, and ultimately caused to rotate with the copper disc. Up to the present we have spoken of two distinct currents being required in the coils A and B, one logging behind the other by a quarter period; and te perform the experiment we might employ an alternating current dynamo with two circuits in the ermature, so arranged with regard to each other es to produce the desired lag. It is, however, possible to perform the experiment by using an ordinary alternating current machine with only one circuit on the ermature, and to produce the second current by means of is. transformer. The current from the machine is sent through the primary of the trans- former and then through the coil A, whilst the secondor current obtained from the transformer is sent through the coil B. It is then possible, by the interposition of a suitable inert resistance into the secondary circuit, to obtain very nearly the desired lag of 90° between the currents in the two coils. As the strength of the secon- dairy current will, by the insertion of the resistance, be considerably reduced, it is necessary, in order to obtain approximately equal Holds from the two coils, to employ a larger number of turns in the coil B than in A. Another method of producing the desired result con- sists in using a transformer with one primary and two secondary coils, and to insert into the circuit of one of the secondary coils a large self induction, and into that of the other a large inert resistance, by which means the desired lag between the two secondary currents can also be obtained. It will be seen that in either of these ways it is possible, by means of a simple alternating current and a. transformer, the coils of which are abreast, to obtain B. revolving magnetic Held of approximately uniform intensity, by which phenomena can he produced in every respect similar to those obtainable from n revolving permanent steel magnet. In the first experiment made by Professor Ferraris, a Gaulerd X: Gibbs transformer, with an equal number of turns in the primary and secondary coils, was used. In the primary circuit was included the coil A, consisting of a few turns of stout wire, whilst in the secondary circuit were included on adjustable and non-inductive resistance und the coil B, the latter consisting of many turns of fine wire. A small hollow copper cylinder was suspended on nthreed in the centre of the two coils, as shown in Fig. 2. If the current was sent throu h only one of the coils, the cylinder remained at rest; int if currents were sent through beth coils, the cylinder immediately started to revolve, twisting the thread on which it was suspended. A reversing switch was inserted in the secondary circuit, and if the current through this circuit was inverted, the cylinder was immediately brought to rest, and started in the opposite direction. Professor Fei-ruris next substi- tuted a. small iron cylinder for the copper one, and found equal results; but when using an iron cylinder sufficiently large to nearly fill the place within the coils, the exper1~ ment did not succeed so well. He attributes this to the greater self induction which the presence of iron in a args mass necessarily introduces into both circuits. Another experiment was made with a small iron cylinder, composed of n. number of thin discs insulated from each other, and this experiment also succeeded. Foucault currents could not in this case be the cause of rotation, and Professor Ferraris thinks that the rotation is due to hysteresis. A larger apparatus was then constructed, which is indicated in our sketch Fig. 3. lu this ouso thi: copper cylinder, which was mounted upon a horizontal axis in bearings, had zu, diameter of 3§in., and was Tin. long, its weight being lllb. This cylinder was closely surrounded bythe primary and secondary coils wound upon wooden bobbins. To make room for the shaft each coil was in halves, the primary consisting of ninety~six turns of seventy-seven mils. wire, both coupled in series, so as to produce the ellbct of a coil of 192 turns; the secondary con5'~

3"5“5f”i°5» 18th May, isss. sos ELECTRO-DYNAMIC ROTATION BY MEANS OF ALTERNATING CURRENTS. Pnorssson GALILEU FERRARIS, of Turin, who, ns will be remembered, was one of the first to make a strictly scientific investigation of transformers, and \vho has since then continued to devote attention to alternating currents, has just published the results of some experiments mnclu with n view to produce continuous rotation by their means. The experiments are interesting, because the apparatus used was bnsed upon an entirely new principle, which muy be capahle of further development, Tho question how to produce continuous rotation by means of nltemating currents is just now occupying the minds of many electricians, because the production of an alter- nating current motor would greatly increase the earning capacity of central electric light stations suppl ing ulter- nuting currents. \Vhethor the apparatus disvised hy Professor Ferraris, and which we are about to describe, will lead to the discovery of an altenniting current motor, is u. question on which we do not pretend to prophesy; but as the princi le involved may also have other applications, notabl)y in the construction of meters for measuring the supply of electricity, wo think it of snllicient importance to lay a short abstract of thc investigation before our renders, Thu principle on which the apparatus is based is exceedingly simple, und will be understood by reference to the accompanying sketches. Lot in Fig.1 the lines 0 A and OB represent in direction and magnitude the strength of two magnetic lields. The combined actin ‘ of these fields on it unit pole, placed in the point O, would then be recprescnterl lay tho resultant 0 R. Instead of having two istinct ficl s, we have one resultant field, represented in direction und nnignitucle by this line. Ii both lields undergo periodic variations of the saino period, the resultant field OR will nlso undergo a riodic varintion, and the point R will trn.\'erse ri. curve, tli; form of which depends upon the precise nnmner in which the lines 0 A and O B change with the time. The fields OA and OB can be produced by two coils, luwing the lines O X and 0 Y respectively for their nxes, nnd which coils are traversed hy nltcrnnting currents of the siuuc period. If _tho nlturnnting currents may he expressed cs sine functions of the time, and if there is an npprecinblo lug between the currents in the two coils, the point R describes an ellipse round O as centre. ln