N DAM L:NTAI. PRINCIPLES. To understand the fundamental principles involved in the opera tion of dynamo-electrie machines, consider first two magnetic poles, N and S. of opposite polarity placed near to each other, as in Fig. 1. Between the poles N and S is a field of magnetic force composed of so-called lines of magnetic force which may lie pictorially indicated by parallel lines, as is done in the illustration. If a eon duet or. for example a round' copper bar C. is placed in the magnetic field, with its axis hori zontal and perpendicular to the lines of force. and is raised and lowered along the path C D so as to cut the lines of force, an electro-motive force is set up or induced in the conductor. The magnitude of the electro-motive force produced in the conductor depends upon the rate at which the lines of force are cut. Due to this electro motive force, one end of the conductor is raised to a higher electric potential than the other, in consequence of which there is a tendency for electricity to flow along the conductor, and if its two ends are electrically connected exterior to the magnetic field so as to make a closed circuit, a current will glow through this circuit. If the gap between the poles and S were infinite in extent in the direction C D, and if the conductor C in its motion along this infinite path were supplied with some sort of sliding contacts by which current could be taken to one end and from the other end, the device would possess all the essential features of a generator or of a motor. An equivalent condition is secured (1) if the poles N and S are made annular in cross section; (2) if the conductor C is arranged so as to rotate as a radius on the common axis of N and S; and 13) if sliding contacts arc pro vided at the centre of rotation nod at the ex tremity of the conductor outside of the polar forces: the device then becomes a homopolar gen erator or motor.
The form of machines just described is note worthy chiefly because of its simplicity and the fact that it was the first form to be invented. All commercial machines now in use operate in such a manner that the conductor moves alter nately forward and backward through the field of force. To illustrate, if the conductor C (Fig. 2) be attached to the circumference of a cylinder or other round body, which rotates about an axis A perpendicular to the paper, it will cut the lines of force by alternate down ward and upward motions. .An electro-motive force in one direction will be generated in the conductor C as it is going up and in the oppo site direction as it is coming down. If the ends
of C are brought down to the shaft A and formed into rings around it, brushes bearing on these rings, if connected to an external circuit, will receive an alternating current, that is, one which flows first in one direction, then in the opposite direction. Machines of this descrip tion are called alternators; the rotating part is called the armature; the pole pieces. N and S. with the remaining magnetic circuit, are called the field, and the rings around the shaft are called collector rings. If the current collected is to flow always in the same direction in the circuit which is external to the machine, sonic device must be provided to change automatically the connection between the armature circuit and the external circuit. Such a device is called a commutator. Its operation may be explained as follows: If in Fig. 2 the ends of the conductor C are brought down to the shaft A and ex tended across to the opposite side of the shaft, and then parallel to it so as to make the corm phite loop shown by Fig. 3, it will he obvious that the eleetro-motire forces generated in the two sides of the loop will he opposite in absolute direction, as indicated by the arrows, but in the same direction as regards the conductor itself. if now the two ends m and n of the loop ;ire attached to two half rings upon which brushes, b and b composed of thin strips of metal or other conducting mate rial rest, and these brushes are placed at such points that they pass from one half ring to the other at the same moment that the conduetor C ceases cutting lines of force in one direction and confluences to cut them in the other direction, the current in the external circuit c will always be iu one direction. Alaehines 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 he represented by the ordinates or vertical distances from the axis it X eurved line ABCD iu Fig. 4, of which the forces acting in the first coil. Likewise if many coils a re employed, the maximum electromotive forces generated in the several coils will occur at different instants, and if these coils are so connected that their eleetro-motive forces are added to one another. the fluctuations will ize each other and the rent obtained will be more uniform.