BYNAMO-ELECTRIC MACHINERY. 565 DYNAMO-ELECTRIC MACHINERY. two sides of the loop will be opposite in absolute direetion. as indicated by the arrows, but in the same direction aa regards the conductor itself. If now the two ends hi and n of the loop are attached to two half rinjis upon which brushes b and 6 composed of thin strips of metal or other conducting mate- rial rest, and t these brushes are '; placed at such f, I points that they pass from one half rinfr to the Fio. S. other at the same moment that the conductor C ceases cutting lines of force in one direction and commences to cut them in the other direction, the current in the external circuit c will always be in one direction. Machines of this description are called direct-current machines. When the machine is operated as an alternator as first described, the direction and intensity of the cur- rent may be represented by the ordinates or vertical distances from the axis O X of the curved line A B C D in Fig. 4, of which the abscisstp 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 forces acting in the first coil. Likewise if many coils are employed, the maximum electromotive forces generated in the several coils will occur at dilTerent instants, and if tliese coils are so connected that their plcitro-molivc forces are added to one another, the fluctuations will neutral- ize each other and the cur- rent obtained will be more uniform. Thus far in this discus- sion it has been assumed that a magnetic field was already provided without inquiring in what manner it was provided. The man- ners in which such fields may be provided will now be described. In the earli- est machines, and in some used even now for medical purposes and in the ring- 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-
Fig. 6. Fio. 6. one direction represented by the cur'e above the axis and has an intensity rising from 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 cune below the axis, and has an intensitj' increasing from nothing at C to a maximum at D and then decreasing to nothing again at E. ^Mien the machine is operated as a direct-cur- rent machine the current produced may be represented by the cur'e A B 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 be noted that the curs'e 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 fact that there is but one rotating coil, as shown by ?'ig. .3. Were another coil at right angles to the first to be added to the armature, the maxi- mum electro-motive forces in it would occur at the moments when there were no electro-motive Fig. 7. siderable size, however, the older form of perma- nent magnet is replaced by an electro-magnet. 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 will 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 a direct-
