[Illustration:  FIG. 3A]

The winding is compound, and in such a direction that the two opposite horizontal poles have the same polarity; it follows from this that there will be two consequent poles in the iron, these being opposite in name to the horizontal poles and at right angles to them, viz., above and below the armature.  Opposite sections of the commutator are connected together internally as in most four-pole machines, so that only two brushes are necessary, at 90 deg. apart.

The section of iron in the field is 60 square inches and rectangular in form, and the whole machine measures 4 ft. 3 in. in length, and 2 ft. in height, without including the height of the bed plate.  The armature is 17 in. in length and the same in diameter, measured over the winding, and develops at the machine terminals 70 volts and 200 amperes at 480 revolutions.  The moving parts of the engine are well balanced, and run remarkably well and without noise at this high rate of speed.

This dynamo serves to develop power to run a motor in an adjoining inclosure, containing some fine specimens of lathes and machine tools constructed by the Oerlikon Works.  These are driven by the motor through the medium of a countershaft, and the power and speed are controlled from the switch board seen at the left of the exhibit, and in Fig. 11.  The resistance, R1, serves to vary the intensity of the shunt field of the dynamo, the volts being indicated by the voltmeter V1, and a resistance separate from the switch board is inserted in the main circuit of the two machines.  The ammeter, A2, is directly connected to the dynamo, and therefore indicates the current, whatever circuit this machine is running.

[Illustration:  Figs. 5-9, 11 plus THE PARIS EXHIBITION—­STAND OF THE OERLIKON WORKS.]

A larger combined engine and dynamo, seen in the center of the stand, serves to run the lighting of the galleries.  The engine is a 60 horse power compound, running at 350 revolutions, and fitted with a governor on the fly wheel, like that described above.

The dynamo is a two-pole machine, the upper pole and yoke being cast in one, and the lower pole, yoke, and combined bed plate forming a separate casting.  The two vertical cores, over which the field bobbins are slipped, are of wrought iron, and are turned with a shoulder at either end, the yokes being recessed to fit them exactly.  The cores are then bolted to the yokes vertically from the top and horizontally below.  The field of this machine is shunt-wound, and in order to maintain the potential constant a hand-regulated resistance—­R2 on the switch board—­is added in circuit with the shunt field.  The voltmeter, V2, immediately above this resistance, serves to indicate the difference of potential at the machine terminals.  Both voltmeters are fitted with keys, so that they are only put in circuit when the readings are taken.