coil resistance is neglected. The resistance of the armature-coil is always very small, and may practically be neglected, so that the current flowing in the system may be considered proportional to the resistance of the circuit outside the machine. In an electric-light installation it would be practically proportional to the number of lamps in use at any time.
In a well-constructed dynamo, running at a given speed and intended to produce current for a certain number of lamps, the pressure should remain constant, whether one lamp or more is in use upon the circuit; and this is of great importance, for otherwise the light from the lamps will vary. When such self-regulation cannot be obtained, electrical governors are employed to secure constant pressure.
One form of alternating-current dynamo consists of a number of coils placed upon the periphery of a wheel, which are revolved before electro-magnets excited from a small continuous-current machine such as that which has just been described. The arrangement is such that an alternating current is produced in the moving coils. This current is collected by means of brushes, as in the last case; but the commutator consists of two rings of metal insulated from each other and from the machine, in connection, however, with the moving coils. The con- ditions for regular pressure and quantity of current produced are the same as in the last case. These descriptions of the two most usual types of dynamo are only of the most superficial kind, the aim being to give merely a general idea of their mode of action.
Having dealt with the production of the electric current, it becomes necessary to consider how it is conveyed to those points where it is required. After this, the apparatus used for obtaining practical results may be examined. In order that the current may travel, the circuit must be complete. The circuit may be compared to a system of hot-water pipes such as are used for warming hot-houses. In a hot-water system there are a boiler, a flow and a return circulating pipe, and pipe-coils at various points for giving off heat at places where warmth is required. In an electrical installation the dynamo replaces the boiler, flow- and return-pipes are represented by the two conducting mains, and the pipe-coils by lamps, motors, and other apparatus. In a hot-water system, it is perfectly evident that the quantity of water passing through every part of the main must be the same. It is so with the electric current: whatever may be the quantity of the current starting from the dynamo, the same quantity comes back to it. But it is not so with the pressure: this diminishes in proportion to the work the current does; consequently the pressure diminishes as the current advances on its path. The passage of a current through a conductor cannot be effected without loss,—i.e., diminished pressure. Loss means work done. If this work has a useful purpose, it is not a loss in the common sense of the word; but in all other cases it is waste. Any pressure of the current lost in a lamp produces a desired result. Since this is not so for the mains, the system must be constructed in such a manner that, except where practical results are needed, as little loss as possible shall occur in conducting the current from point to point,—which is effected by making the mains as large as possible. The size of the mains is
