On the top of the magnet yoke is the electrical control governor, Fig. 4.  It consists of one moving spindle on which are keyed a small soft iron bar, and also a double finger, T. There is also a spiral spring, X, attached at one end to the spindle, and at the other to an adjustable top head and clamping nut, Y. The double finger, T, covers or opens a small hole in the face, U, communicating by the pipe, W, to the diaphragm, L. The action of the magnet yoke is to attract the needle toward the poles of the magnet, while by turning the head the spiral spring, X, is brought into tension to resist and balance this force, and can be set and adjusted to any degree of tension.  The double finger, T, turns with the needle, and, by more or less covering the small air inlet hole, U, it regulates the access of air to the regulating diaphragm, L. The second finger is for safety in case the brushes get thrown off, or the magnet circuit be broken, in which case the machine would otherwise gain a considerable increase of speed before the diaphragm would act.  In these cases, however, the needle ceases to be attracted, falls back, and the safety finger closes the air inlet hole.

There is no resistance to the free movement of this regulator.  A fraction of a volt increase or decrease of potential produces a considerable movement of the finger, sufficient to govern the steam pressure, and in ordinary work it is found possible to maintain the potential within one volt of the standard at all loads within the capacity of the machine, excepting only a slight momentary variation when a large portion of the load is switched on or off.

The resistance of the armature from brush to brush is only 0.0032 ohm, the resistance of the field magnets is only 17.7 ohms, while the normal output of the dynamo is 200 amperes at 80 volts.  This, excluding other losses, gives an efficiency of 97 per cent.  The other losses are due to eddy currents throughout the armature, magnetic retardation, and bearing friction.  They have been carefully measured.  By separately exciting the field magnets from another dynamo, and observing the increased steam pressure required to maintain the speed constant, the corresponding power was afterward calculated in watts.

The commercial efficiency of this dynamo, after allowing for all losses, is a little over 90 per cent.  In the larger sizes it rises to 94 per cent.  Assuming the compound steam turbine to give a return of 70 per cent. of the total mechanical energy of the steam, and the dynamos to convert 90 per cent of this into electrical output, gives a resulting efficiency of 63 per cent.  As steam at 90 lb. pressure above the atmosphere will with a perfect non-condensing engine give a horse power for every 20.5 lb. of steam consumed per hour, it follows that an electrical generator of 63 per cent. efficiency will consume 32.5 lb. of steam for every electrical horse power per hour.

Again, with steam at 150 lb. pressure above the atmosphere, a generator of the same efficiency would consume only 22.2 lb. of steam per electrical horse power per hour.