Regulation can also be effected by governing the steam-engine, so that its speed is exactly proportional to the load. In Richardson's electric governor (Fig. 24) the valve which admits steam to the engine is a double-beat equilibrium valve E; its stalk passes upward and is acted upon by a plunger /4, which is pressed down the shorter arm of a lever 1,, which is in turn connected with a long vertical spindle having a weight C at its lower end. and at its upper end carrying the iron core B, surrounded by the solenoid A. A spring Seounterpoises the slight upward pressure of the steam on the valve. When the cur rent passes through the solenoid A it lifts the core B to a certain height, and admits to the engine a sufficient quantity of steam to drive the en gine at the speed requisite to maintain the current. Should the resist ance of the circuit be increased by the introduction of additional lamps, the core B will fall a little, thereby turning on more strain, until the speed has risen to that now necessary. For additional safety a separate electromagnet a is added, which, when in action, holds up the heavy iron block b. Should the circuit from any cause be broken, the block b instantly descends and cuts off the steam. Similar engine-governors have been devised by Winans, Jamieson, and others. Further informa tion respecting electric governors, and their actual applications in vari ous installations of electric-lights, may be found in the following papers: A. Jamieson, Electric-Lighting for Steamships, Proc. Inst. Ediurs., vela I.xxix, session Part I : F. W. Willans, The Electric Regula tion of the Speed of Steam-Engines, Proc. hist. Cie. Engrs., vol. lxxxi, session 1884-'85, Part III. (See also Thompson's Dynamo-Electric 11a chinery) Dynamo-metric governing has also been proposed, the regulation being effected by the action due to the variation in torque with varying load. Governing by steam-pressure has also been proposed. In this system the steam-pressure is kept constant, and equal quanti ties of steam admitted in each stroke between the speed. 11 principles chore enumerate l have been carried out in a large number of designs of machines, each having its special peculiarities and advantages. For the sake of obtaining a better understanding of the vari ous types, we have in the following grouped dynamo-electric generators primarily into two divisions: Continuous-current and alternate-current dynamos.
1. Comsuotts-CennErr DYNAmos.—These may be divided into (a) constant current and (b) constant potential ; the former were those first brought to praatieallwrfection, and hence we shall take them in that order, giving examples of the machines in most general use, together with the. regulating methisle employed for keeping the current constant.
The Brush Arc-Light Jlarhine.—The most characteristic feature of the Brush machine (see p. 527, old edition) lies in the form and construction of its armature, which consists of a built-up iron ring. the cross-section of which is generally rectangular, lint in the direction of its circumference it is alternately wide and narrow, as shown in Fig. 25, which represents the iron armature-ring and explains its constrnction. On reference It this figure it will be seen that the ring is divided up into as many sectors as there are hi)bbins to be wound by it number of rretuixgulnw depressions or grooves: in these the coils of in.uIatt1 eopper wire are
wound until 1 he groove is filled up, and the flat, converging recesses become flush with the face of the intermediate thicker portions or pole-pieces. by which they are separated front one another, All the coils are, like those in the Gramme machine, wound in the samc direction. Fig. 26 is a diagram illustrative, not only of llo• distribution of the coils around the ring, but of the method by wideh the connections are made the inner ends of each of the coils is con nected by it Wirt' to the iintor end of the corresponding coil, at the opposite end of the saute of the ring. mid the outer ends of all the vas are brought through the shaft of the machine, and are connected to corresponding portions of the commutator, where the currents are collected by sunahly placed copper plates. Referring to the diagram, it will be that the inner end Al of the mil I is connected to At whieh is the inner end of the coil 3; it is c.tnneeteiI to A', to At, and at on round the ring, and the outer Bt IP, etc., arc connnected to the commutator by conducting wires insulated from one another. The two free ends of each pair of diametrically opposed' coils are, after passing through the shaft of the machine, attached respectively to two diametrically opposite segments of the same com mutator, which segments are insulated from one an other and from any other pairs of coils. The com mutator which is attached to and rotates with the driving-shaft of the machine consists of a set of separate copper rings or flat cylinders, of which there are as many on the shaft as there are pairs of coils on the armature, and each of these cylinders consists of two segments insulated from one another on one side of the shaft by a small air-space about in. wide, and on the other by a piece of copper separated from the segments by two smaller air spaces. The ar rangement is shown in Fig. 27, in which A and B are the two segments connected, re spectively, to corresponding coils on oppo site sides of the armature, and attached by an insulating material to the shaft S: C is the copper insulat ing piece, the object of winch is to separate either of the flat copper brushes or collectors, which press upon the periphery of the commutator, from either of the segments during the interval occupied by one pair of coils passing the vertieal, or, in other words, through the neutral portion of the magnetic field ; this occurs twice in each revolution of the arma ture, and therefore of the commutator. At the time when any pair of bobbins is in this way cut out of the general circuit, their own circuit is open, so that no current can circulate or be induced in them. By this arrangement each pair of coils has in succession, in each revolu tion, a period of rest equal to one quarter of a revolution, and has a current passing through it fur only 75 per cent. of the time the machine is running ; to it is in a great measure due the very small development of heat in the working of the Brush maehine ; and it presents also another important element of efficiency to the machine, namely, that each pair of bobbins as it passes the neutral portion of the magnetic field, and is therefore incapable of doing work and contributing eleetni-motive force to the general current, is itself cut out. of the circuit.