Page:Electricity (1912) Kapp.djvu/204

200 current is led round the exciting coils in the proper sense, this feeble current will slightly increase the original magnetisation, this in turn will produce a slightly greater e.m.f., this again will strengthen the field, and so on until the machine is fully excited. We have here the principle of "self-excitation." Once the machine has been excited there remains what may be called "residual magnetism," and the process of self-exciting takes place more readily. A machine in which the whole of the armature current is led round the exciting coils is called a "series machine," which term is chosen to indicate, that the exciting coils are coupled in series with the external circuit. Since the whole of the current is used for .excitation a moderate number of turns in the series coils suffices. But these must be of sufficiently stout wire to carry the whole of the current.

Now let us wind these coils with much finer wire, but, to make up for the lesser current which such wire can take, let us use a much larger number of turns. The current which we wish to have in the external circuit is much too large to be carried by this fine wire coil, and we must therefore feed it directly from the brushes. The exciting coil