December 3, 1891] 



NATURE 



107 



three wires, a, b, c (Figs. 20, 21, 22), each current was at 

 any moment algebraically equal to the sum of the other 

 two. To test, therefore, whether the currents flowing in 

 the three parallel wires between Lauffen and Frankfort 

 fulfilled this condition, we had merely to find out whether 

 any current was induced in a neighbouring telegraph 

 wire which was sufiiciently far away as to be practically 

 at the same distance from each of the Lauffen- Frankfort 

 wires. 



Between Frankfort and Hanau the power wires are 

 carried on one side of a broad railway, and for some 

 eight or nine miles the telegraph wires run on the other 

 side ; the telegraph wires for the remainder of the distance 

 between Frankfort and Hanau following quite a different 

 route. If one of these telegraph wires were put to earth 

 at Frankfort and at Hanau, and if a telephone were 

 placed in the circuit, a confused chattering of telegraph 

 instruments was always heard in this telephone, due to 

 induction from the telegraph lines on the same posts. 

 But during the hours that power was being transmitted 



ways, leaving the field magnet in position, as seen in 

 Fig. 34. Each of the 32 flat-looking plates round the 

 circumference of the field-magnet is a magnetic pole, 

 the poles being alternately north and south. This result 

 is attained by constructing the field magnet in the in- 

 genious manner shown in Fig. 35, the coil which carries 

 the direct current to magnetize this field magnet being 

 wound in the circumferential channel seen in section in 

 Fig. 35. 



The armature bars, 96 in number, are constructed of 

 copper rods 29 mm. in diameter, insulated in asbestos 

 tubes, and slipped through holes (parallel to the axis of 

 rotation) punched out of the laminated iron ring which 

 composes the armature core ; this burying of the con- 



Field magnet of the Lauflfen three-phase alternate current dynamo. 



through the wires on the other side of the broad railway 

 a rhythmical hum could be detected superimposed on the 

 confused babel of telegraph signals, proving that the 

 three alternate currents were either not truly sine cur- 

 rents, or that their phase difference was not accurately 

 120". 



To generate the three-phase current at Lauffen, the 

 extremely compact dynamo shown in Fig, 33 was de- 

 signed by Mr. Brown, and constructed at the Oerlikon 

 \yorks, near Zurich. The armature is wound with three 

 distinct circuits, each arranged to give 1400 amperes at 

 a potential difference of 50 volts, so that the dynamo can 

 develop 300 horse-power. To avoid, as far as possible, 

 rubbing contacts, the armature remains stationary and 

 the field magnet revolves ; while by the employment of 

 32 poles a frequency of 40 complete alternations per 

 second ran be obtained in each circuit when the field 

 magnet only makes 150 revolutions per minute. 



For examining the interior, the armature, which forms 

 the outside shell of the machine, can be withdrawn side- 



NO. 1153, VOL. 45] 



Fig. 35. — Section of the field ma^n^t of the Liuffen dynamo. 



ductors to a small depth in the iron being, as already ex- 

 plained in the case of the Dobrowolski motor, for the 

 purpose of avoiding Foucault currents being induced in 

 the thick copper bars. 



A portion of the three separate windings, aaaa,bbbb, 

 cc cc, on the armature is shown in Fig. 36, which repre- 

 sents a bit of the circumferential part laid out flat ; the 

 dotted rectangles indicate the poles, and to avoid con- 

 fusion the armature bars, parallel to the length of the 

 poles, are drawn longer in proportion than they really 

 are. 



In order that the electromotive forces induced in all the 

 up and down bars of any one of the windings aaaa in 

 Fig. 36 should help one another, the distance between 



Fig. 36. — Portion of the armature-winding of the Lauffen three-phase 

 alternate current dynamo. 



any up and the adjacent down conductor of the same 

 winding must be equal to the distance between two 

 adjacent poles — that is, to 1/32 of the circumference of 

 the armature ; and in order that the electromotive force 

 generated by the winding bbb b should differ in phase by 

 120° from the electromotive force generated in the wind- 

 ing a a a a, the distance between an up bar of the winding 

 aaaa and the following up bar of the winding bbbb 

 must be two-thirds of the distance between the centres of 

 two adjacent poles— that is, must be 1/48 of the circum- 

 ference of the armature. Similarly, an up bar of the 

 winding cccc must be behind the preceding up bar of 

 the winding cccc by 1/48 of the circumference of the 

 armature. 



