October 29, 1891] 



NATURE 



617 



by a suspended compass needle, the point of the arrow 



indicating the north-seeking pole of the compass needle. 



If in place of the suspended compass needle there be a 

 piece of copper, currents will be induced in this copper by 

 the rotating magnetic field, tending to make the cylinder 

 follow the field. Hence, if the copper take the form of a 

 cylinder, with its axis coinciding with the axis of the ring, 

 and supported so that it can rotate about this axis, the 

 cylinder will run after the rotating field until it catches it 

 up, when thetwo will move nearly synchronously together. 

 On applying a resistance to the rotation of this cylinder — 

 that is, on making the motor do work — the speed of the 

 cylinder will be checked, but a small diminution of speed 

 will cause large currents to be induced in the copper, 

 and a pulling force to be exerted between the rotating 

 field and the lagging cylinder, tending to drag the cylinder 

 round. Hence this arrangement of Prof. Ferraris produces 

 not merely a self-starting alternate current motor, but 

 one which runs almost synchronously with the dynamo for 

 wide variations in the load, and which has neither com- 

 mutator, rubbing contacts, brushes, nor the possibility of 

 sparking. 



Within the past few weeks we have learnt that the 

 idea of obtaining a rotating magnetic field was mentioned 

 by M. Marcel Deprez, in a French patent dated May 



the copper cylinder originally used by Prof. Ferraris was 

 next made hollow, and the interior filled with soft iron, 

 the iron being laminated in planes at right angles to the 

 axis, to prevent currents being induced in the iron ; and to 

 make the currents induced in the copper cylinder follow 

 the most useful path the next step was to make a num- 

 ber of cuts through the hollow copper cylinder parallel to 

 the axis of rotation. Practically, then, the rotating por- 

 tion becomes a laminated cylinder of iron, on which is 

 wound insulated wire parallel to the axis, as in a 

 Siemens armature, but with this difference, that all 

 the wires are electrically joined together at each end 

 of the cylinder. 



A two-phase alternate current motor was constructed 

 and used by Pi of Ferraris in his laboratory at Turin in 

 1885. But not appreciating the practical importance of 

 his own invention, and thinking that no motor requiring 

 more than two wires could interest anyone but the natural 

 philosopher. Prof Ferraris occupied himself with attempts 

 to utilize the rotatory magnetic field in measuring the 

 resistance of conductors and with mathematical investiga- 

 tions on alternate currents. It was not, therefore, until 

 the spring of 1888 that the results of his researches were 

 published ; when, a few months later, commercial motors 

 based on exactly the same principles were brought oi'j 



A Sin (-e) 



m 





A Cos(-e) 



m 



Fig. 13.— Schuckert two-phase alternate current generator and transformer. The arrows indicate the actual direction of the currents for the position of 



the armature shown. 



1883. In that patent, when speaking of the magnetic 

 field produced by the current flowing round a Gramme 

 ring, he says : " Cetie rotation du champ mag7tctique pent 

 ttre obtenue sans /aire mouvoir aucune piece ; pour cela 

 on /era naitre le champ a I aide de deux courants dont les 

 points d' entree sotii sur deux diamitres perpettdiculaires ; 

 Vaitnentation de ce champ sera alors une resultante dont 

 la position dtpe7id des intensity relatives des deux courants, 

 ainsi que cela a et^ decrit ci-dessus pour le comparateur 

 des courants ; il suffit de /aire varier le rapport de ces 

 intensities pour faire tourner cetie rtfsultanie, et avecelle le 

 champ magnetigue." 



It does not, however, appear to have occurred to M. 

 Deprez that this rotation of a magnetic field might be em- 

 ployed to induce currents, and thus give motion to a piece 

 of metal placed inside the Gramme ring ; nor does he say 

 anything about two harmonic alternate currents differing 

 by 90° in phase producing the exact variation of current 

 required. Although, then, what may be called the geo- 

 metrical idea of producing a rotating magnetic field was 

 certainly clearly described by M. Deprez, the credit of re- 

 discovering this principle, and, what is far more important, 

 of applying it in the design of the two-phase alternate 

 current motor, is due to Prof. Ferraris. 



To increase the strength of the rotating mignetic field, 



NO. I 148, VOL. 44] 



with considerable eclat by Mr. Tesla, of Pittsburg, who 

 had been working independently in the same direction. 



To produce two alternate currents, differing by 90° in 

 phase, the following device (Fig. 13) may be adopted, 

 and is the one employed by Messrs. Schuckert in trans- 

 mitting power at 2000 volts from the Palm Garden at 

 Frankfort to the Exhibition, and by Messrs. Siemens and 

 Halske for experiments on rotatory field alternate current 

 motors in the Exhibition ; the latter firm, however, 

 not employing the special form of transformer shown 

 symbolically in Fig. 13. In addition to the armature 

 of a Gramme dynamo being joined up in the well- 

 known way with the ordinary direct current com- 

 mutator (this commutator and brushes rubbing on it 

 not being shown in Fig. 13), four points at equal distances 

 on the armature are permanently connected with four 

 metal rings, Ri, R.^ R^j, and R4, which rotate with the 

 armature. Then it is easy to prove that while the 

 machine is producing a direct current, used for exciting 

 the field magnets as well as for any other purpose desired, 

 the current passing through the wires attached to the 

 brushes Bi, B2,and the current passing through the wires 

 attached to the brushes B3, \^^, each alternate very nearly 

 as the sine function of the time, the one reaching its 

 maximum value when the other is nought. 



