The Story of Electricity

by John Munro

Let the right hand rest on the north pole of a magnet and the forefinger be extended in the direction of the lines of force, then the outstretched thumb will indicate the direction in which the wire or conductor moves and the bent middle finger the direction of the current. These three digits, as will be noticed, are all at right angles to each other, and this relation is the best for inducing the strongest current in a dynamo or the most energetic movement of the conductor in an electric motor.

Of course in a dynamo-electric generator the stronger the magnetic field, the less the resistance of the conductor, and the faster it is moved across the lines of force, that is to say, the more lines it cuts in a second the stronger is the current produced. Similarly in an electric motor, the stronger the current and magnetic field the faster will the conductor move.

The most convenient motion to give the conductor in practice is one of rotation, and hence the dynamo usually consists of a coil or series of coils of insulated wire termed the "armature," which is mounted on a spindle and rapidly rotated in a strong magnetic field between the poles of powerful magnets. Currents are generated in the coils, now in one direction then in another, as they revolve or cross different parts of the field; and, by means of a device termed a commutator, these currents can be collected or sifted at will, and led away by wires to an electric lamp, an accumulator, or an electric motor, as desired. The character of the electricity is precisely the same as that generated in the voltaic battery.

The commutator may only collect the currents as they are generated, and supply what is called an alternating current, that is to say, a current which alternates or changes its direction several hundred times a second, or it may sift the currents as they are produced and supply what is termed a continuous current, that is, a current always in the same direction, like that of a voltaic battery. Some machines are made to supply alternating currents, others continuous currents. Either class of current will do for electric lamps, but only continuous currents are used for electo-plating, or, in general, for electric motors.

In the "magneto-electric" machine the FIELD MAGNETS are simply steel bars permanently magnetised, but in the ordinary dynamo the field magnets are electro-magnets excited to a high pitch by means of the current generated in the moving conductor or armature. In the "series-wound" machine the whole of the current generated in the armature also goes through the coils of the field magnets. Such a machine is sketched in figure 40, where A is the armature, consisting of an iron core surrounded by coils of wire and rotating in the field of a powerful electro-magnet NS in the direction of the arrows. For the sake of simplicity only twelve coils are represented. They are all in circuit one with another, and a wire connects the ends of each coil to corresponding metal bars on the commutator C. These bars are insulated from each other on the spindle X of the armature. Now, as each coil passes through the magnetic field in turn, a current is excited in it. Each coil therefore resembles an individual cell of a voltaic battery, connected in series. The current is drawn off from the ring by two copper "brushes" b, be which rub upon the bars of the commutator at opposite ends of a diameter, as shown. One brush is the positive pole of the dynamo, the other is the negative, and the current will flow through any wire or external circuit which may be connected with these, whether electric lamps, motors, accumulators, electro-plating baths, or other device. The small arrows show the movements of the current throughout the machine, and the terminals are marked (+) positive and (-) negative.

It will be observed that the current excited in the armature also flows through the coils of the electro-magnets, and thus keeps up their strength. When the machine is first started the current is feeble, because the field of the magnets in which the armature revolves is merely that due to the dregs or "residual magnetism" left in the soft iron cores of the magnet since the last time the machine was used. But this feeble current exalts the strength of the field-magnets, producing a stronger field, which in turn excites a still stronger current in the armature, and this process of give and take goes on until the full strength or "saturation" of the magnets is attained.

Such is the "series" dynamo, of which the well-known Gramme machine is a type. Figure 41 illustrates this machine as it is actually made, A being the armature revolving between the poles NS of the field-magnets M, M, M' M', on a spindle which is driven by means of a belt on the pulley P from a separate engine The brushes b b' of the commutator C collect the current, which in this case is continuous, or constant in its direction.

The current of the series machine varies with the resistance of the external or working circuit, because that is included in the circuit of the field magnets and the armature. Thus, if we vary the number of electric lamps fed by the machine, we shall vary the current it is capable of yielding. With arc lamps in series, by adding to the number in circuit we increase the resistance of the outer circuit, and therefore diminish the strength of the current yielded by the machine, because the current, weakened by the increase of resistance, fails to excite the field magnets as strongly as before. On the other hand, with glow lamps arranged in parallel, the reverse is the case, and putting more lamps in circuit increases the power of the machine, by diminishing the resistance of the outer circuit in providing more cross-cuts for the current. This, of course, is a drawback to the series machine in places where the number of lamps to be lighted varies from time to time. In the "shunt-wound" machine the field magnets are excited by diverting a small portion of the main current from the armature through them, by means of a "shunt" or loop circuit. Thus in figure 42 where C is the commutator and b b' the brushes, M is a shunt circuit through the magnets, and E is the external or working circuit of the machine.

The small arrows indicate the directions of the currents. With this arrangement the addition of more glow lamps to the external circuit E DIMINISHES the current, because the portion of it which flows through the by-path M, and excites the magnets, is less now that the alternative route for the current through E is of lower resistance than before. When fewer glow lamps are in the external circuit E, and its resistance therefore higher, the current in the shunt circuit M is greater than before, the magnets become stronger, and the electromotive force of the armature is increased. The Edison machine is of this type, and is illustrated in figure 43, where M M' are the field magnets with their poles N S, between which the armature A is revolved by means of the belt B, and a pulley seen behind. The leading wires W W convey the current from the brushes of the commutator to the external circuit. In this machine the conductors of the armature are not coils of wire, but separate bars of copper.

In shunt machines the variation of current due to a varying number of lamps in use occasions a rise and fall in the brightness of the lamps which is undesirable, and hence a third class of dynamo has been devised, which combines the principles of both the series and shunt machines. This is the "compound-wound" machine, in which the magnets are wound partly in shunt and partly in series with the armature, in such a manner that the strength of the field-magnets and the electromotive force of the current do not vary much, whatever be the number of lamps in circuit. In alternate current machines the electromotive force keeps constant, as the field- magnets are excited by a separate machine, giving a continuous current.