Thus far in this sion it has been assumed that a magnetic field was already provided without inquiring in what manner it was provided. The ners in which sneh fields may be provided will now be described. In the est machines, and in sonic used even now for medical purposes and in the ing magnets of telephones, the field is provided by a permanent magnet which may be of the form shown by Fig. 6. In all machines of any con abscissas or distances along the axis represent time. To illustrate, the distance A E represents the time of one complete revolution of the con ductor. For half this time the current is in one direction represented by the curve above the axis and has an intensity rising front noth ing at A to a maximum at B, and then falling to nothing at C. For the other half of this time the direction of the current is represented by the curve below the axis, and has an intensity increasing from nothing at C to a maximum at D and then decreasing to nothing again at E. When the machine is operated as a direct-cur rent machine the current produced may be represented by the curve A is C D E in Fig. 5, of which the ordinates and abscissae have the same significance as in the curve previously ex plained. It will he noted that the curve being always above the axis shows the current to flow always in one direction, but to fluctuate in in tensity. This excessive fluctuation is due to the fart that there is but one rotating (-oil, as shown by Fig. 3. Were another coil at right angles to the first to be added to the armature, the maxi mum eleetro-motive forces in it would occur at the moments when there were no electro-motive siderable size, however, the older form of perma nent magnet is replaced by an electromagnet.
The method of producing such a magnet is indi cated diagrammat ically by Fig. 7. in which the cast-iron or steel yoke A is wound with a coil of wire, a, a. If a current flows through the wind ing in the direc tion indicated by the arrow, mag netic poles will be produced at N and S and a magnetic field exist be tween them. The parts of the mag net may differ much in form and arrangement, but a magnetic circuit A N S interlinked or wound with an electric circuit (a a) is an essential feature. The electric current for exciting the field may be furnished from a source exterior to the generator or motor, or, in the case of at direct current dynamo, by the machine itself. There
are two ways in which this self-excitation may be accomplished. I f current is taken from the positive brush and led direct ly through the mag net-winding and back to the negative brush. as shown by Fig. S. the magnet is in parallel or shunt with the external cir cuit, and the ma chine is called shunt wound. In a shunt wound machine only part of the current generated pa s s e through the magnet winding. If, how- ever. all of the current coining from the armature passes through the field-magnet and then goes to the external circuit. as shown by Fig. 9, the ma chine is called se ries wound. if both shunt and se ries windings are employed, the ma chine is defined as Compound wound.
Compound wind ings are of two classes, defined as long shunt and short shunt. In the former the cur rent used in the shunt windings is a lso pa seed through the series winding along with the main current, as shown by Fig. 10. hi the latter the current from the shunt coils passes directly back to the arma ture, avoiding the series turns, as shown by Fig.
II. The field-magnets of alternators are excited by a current from a source exterior to the machine, usually from an auxiliary direct-current dyna mo, called the ex citer. The relative arrangement of the field - magnet wind ing and the exterior circuit in separately excited generators is by Fig. 7. The machines shown in these various dia grams, and in fact - all the machines so far mentioned, have had but two mag netic poles. It is evident., however, that there may be as many pairs of poles as is desired. so long as the armature connections :ire arranged to correspond. Machines having only one pair of poles are called bipolar machines; machines hav ing, two or more pairs of poles are called multi polar. Most machines now built in the United States, except those of very small size, are multi polar machines, a typical form being shown in Fig. 12. There are usually as many brushes bearing on the commutator as there are field poles. but this is not absolutely nec essary. since the armature conduct ors may be so con nected as to re quire only two brushes. The lat ter practice is fol lowed in street railway motors.