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 DYNAMO nature. Yet by the aid of the dynamo the power to be derived from waterfalls can be economically and conveniently converted into an electrical form and brought to the neighbouring factory or distant town, to be there reconverted by motors into mechanical power. Over any but very short distances energy is most easily transmitted when it is in an electrical form, and turbine-driven dynamos are very largely and successfully employed for such transmission. Thus by conducing to the utilization of water-power which may previously have had but little value owing to its disadvantageous situation, the dynamo may almost be said to have added another to our available natural resources.

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being determined by the relative directions of the field and movement.5 The second fundamental equation of the dynamo brings to light its mechanical side, and rests on Oerstedt’s discovery of the interaction of a magnetic field and an electric current. If a straight electric conductor carrying a current be so placed in a magnetic field that its length is not parallel to the direction of the lines of flux, it is acted on by a force which, if not balanced by an equal and opposite force, will cause the conductor to move The two essential parts of the dynamo, as required by in a definite direction; or if the conductor is fixed and its definition, may be illustrated by the original disc the magnet be unconstrained, the latter will itself move in machine of Faraday. They are (1) the iron magnet, the opposite direction. Now in the dynamo the inductors between the poles of which a magnetic field exists, and are placed so that their length may be as nearly as possible (2) the electrical conductors, represented by the rotating at right angles to the field; hence when they are rotated copper disc. The sector of the disc 1 cutting the lines of and an electric current begins to flow under the E.M.F. the field forms part of a closed electric circuit, and has an which they induce, a mutual force at once arises between E.M.F. induced in it, by reason of which it is no longer the copper conductors and the magnet, and the direction of simply a conductor, but may be called an “inductor” In this force must by Lenz’s law 6 be opposed to the direction its more highly developed form the simple copper disc of the movement. Thus as soon as the disc of Fig. 1 is becomes a system of many inductors so inter-connected as rotated and its circuit is closed, it experiences a to add up their several E.M.F.’s. Since these inductors are mechanical pull or drag, which must be overcome by the very commonly mounted on an iron structure, which may force applied to rotate it. While the magnet must be be likened to the keeper or “ armature ” of a magnet firmly held so as to remain stationary, the mechanical rotating between its poles, the term “ armature ” has been construction of the armature must be such that its extended to cover not only the iron core, but also the wires inductors can be forcibly driven through the magnetic on it, and is often applied to the copper conductors them- field against the mutual pull. The law of electrodynamic selves even when there is no iron core. In the dynamo of action requires its equation of mechanical force, just as Faraday it is the “ armature ” which is rotated, and such the law of electromagnetic induction requires the equation is usually the case; sometimes, however, the magnet, or a of electromotive force, and it may be expressed in portion of it, is rotated. It is in fact immaterial to the analogous terms. If a conductor of length L cm., carrying action whether the one or the other is moved, or both, so a current C amperes, is immersed in a field of uniform long as their relative motion causes the armature inductors density B, and the length of the conductor is at right (/ to cut the magnetic flux. As to the ultimate reason why angles to the direction of the lines, it is acted on by a force an E.M.F. should be thereby induced, physical science cannot as yet yield any surer knowledge than in the days of F = BffLC x 10_1 dynes, .... (II.) Faraday.2 For the engineer, the simple fact is sufficient and the direction of this force is at right angles to the that the E.M.F. of the dynamo is due to the cutting of conductor and to the field. The rate at which electrical the magnetic flux by the inductors, and, further, is pro- energy is developed, when this force is overcome by moving portional to the rate at which the lines are cut.3 the inductor as a dynamo through the field, is EC = The equation of the electromotive force which is required B^LVC x 10~8 watts, whence the equality of the mechanical in order to render this statement quantitative must power absorbed and the electrical power developed (as contain three factors, namely, the density of the flux in required by the law of the conservation of energy) is easily the air-gap through which the inestablished. The whole of this power is not, however, ductors move, the length of the available at the terminals of the machine; if Ka be the inductors, and the speed of their resistance of the armature in ohms, the passage of the movement. For given values of the 1 current C(t through the armature inductors causes a loss of first and third factors and a single j pressure of CaRa volts, and a corresponding loss of energy straight inductor moved parallel to l -i-L/^ in the armature at the rate of Ca2Ra watts. As the resistitself through a uniform field, the ance of the external circuit Rc is lowered, the current maximum rate of cutting is evidently En. . n Movement, of Jnductor* obtained when the three directions ^ “ p> is increased, but at the same time the external Fig. 2. of the lines, the armature involtage at the terminals of the machine is decreased, until ductor, and the relative motion are respectively at right a maximum output is reached, when the loss of volts over angles to each other, as shown by the three co-ordinate the internal resistance is equal to the loss of volts over the axes of Fig. 2. Under these circumstances the E.M.F. external resistance. The increase of the current is, howof the inductor is ever, accompanied by a progressive increase in the loss of energy over the armature, and as this is expended in E = B!7LV x 10 8 volts, (Iheating the armature conductors, their temperature may 4 where is the density of the lines of flux per square rise so much as to destroy the insulating materials with centimetre in the air-gap, L is the active length of the which they are covered. Hence the temperature which inductor within the field in centimetres, and Y is the the machine may be permitted to attain in its working is velocity of movement in centimetres per second. Further, the direction in which the E.M.F. has the above maximum of great importance in determining its output, the current which forms one factor therein being primarily limited by value is along the length of the inductor, its “sense” the heating of the armature wires. The lower the resistance of the armature, the less the rise of its temperature 1 See Electricity, Ency. Brit. vol. viii. p. 78. I “ On the Physical Lines of Magnetic Force,” EMI. Mag., June for a given current flowing through it; and the reason for 1852. 3 5 Faraday, Exp. Res., series xxviii. § 34, pars. 3104, 3114-15. 4 6 Faraday, Exp. Res., series i. § 4, pars. 114-119. See Electromagnet, New Yolumes, Ency. Brit. Electricity, Ency. Brit. vol. viii. pp. 11 and 76.