There are various forms of the magneto-electric machines, as well as of commutators, but the foregoing shows the general principle of them all.
It will now be necessary to explain the nature of a dynamo-electric machine, called, for shortness, a dynamo, and to show in what it differs from a magneto-electric machine.
I have explained how an electric current is produced by a wire passing in front of a magnet; now, this magnet may either be of the ordinary kind, or it may be what is termed an electro-magnet. One of the effects which electricity can be made to produce is the magnetising of steel bars to form the ordinary and well-known permanent magnets which are used in ships' compasses, &c. To produce this effect, part of the wire in a circuit is made into a spiral as in Fig. 5.
The steel rod to be magnetised is placed within the spiral, and a continuous current of electricity is then sent through the wire, which causes the rod to become magnetised with a North pole at one end, and a South pole at the other. The more current is passed through the circuit, and the more turns are in the spiral, the more quickly and strongly is the rod magnetised; and it will retain its magnetism for an indefinite time if made of suitable steel. There is a point at which the metal is said to be saturated with magnetism, and the strength it has then acquired will be that which it will retain afterwards, although while under the influence of the current that strength may be considerably exceeded. If instead of a steel rod one of iron is placed in the spiral, and the current is passed through as before, it will be magnetised in the same manner; but as soon as the current is stopped, the rod loses almost all its magnetism, and if the current is then passed in the opposite direction the rod will be magnetised in the opposite way. The softer and more homogeneous is the iron, the more instantaneously will it acquire and lose its magnetism, and the greater strength of magnetism it is able to acquire. An iron bar, round which are wound a large number of turns of insulated or covered wire, constitutes an electro-magnet. The difference then between a magneto-electric and a dynamo-electric machine is, that in the former permanent magnets are used, and in the latter electro-magnets take their place. I do not intend to go into particulars as to the construction of the various dynamos in present use, as there are many books to be had in which these machines are fully described. I need merely say that in the so-called continuous-current dynamos, the whole or part of the current produced is made to pass through the coils of the electro-magnets, thus inducing in them the required magnetism. I showed how, in the magneto-electric machine, the currents are collected by means of a commutator, and it is evident that in Figs. 2, 3, and 4 there might be separate wires coming from each bobbin to B and C; and if there were more than two bobbins, there might still be two wires from each to B and C. On the other hand the collecting collar might be split into more sections; in fact there might be as many sections as bobbins. To show how the current is collected in continuous-current dynamos, I must give a short explanation of the revolving part or armature of a standard type of machine.