DYNAMO 447 DYNAMO In all cases the poles of the field- magnet are fashioned so as to embrace as large a portion of the armature cir- cumference, with as small an air-gap, as may be practicable. To effect this, soft-iron cheeks or pole-pieces are com- monly fixed to the shanks which carry the magnetizing coils. Cross-magneti- zation of the poles due to armature re- action is often prevented by a deep nar- row slot across the curved face of each pole-piece, paralleling the direction of the lines of force in the field. There are several methods of arrang- ing for the magnetization of the fields, which it will be convenient to defer until there has been given some description of the armature and its construction. The principle underlying this may best be understood by considering what takes place when a single rectangular frame or loop conductor is revolved about the longer diameter in a magnetic field of parallel lines of force. We may imagine the axis or shaft, about which the rectangular frame is rotated, to cross the field at right angles, so that the two long sides of the rectangle parallel to the axis are continually cutting lines of force as the frame revolves. The two short sides do not cross the lines of force at all, but simply slide through them, and have, therefore, no E.M.F. actually induced in them. They serve only to complete the electrical system, so enabling any E.M.F. induced in the active (long) sides to produce a cur- rent in the system. The conductor must be imagined as insulated from the shaft. At the moment when the frame is at right angles to the direction of the field, no lines of force are being cut, and the E.M.F. induced in both active sides, and consequently the current in the system is zero. As, during the course of a quarter revolution (90°) the plane of the frame becomes parallel to the direc- tion of the field, more and more lines of force are cut, and an E.M.F. of contin- ually increasing magnitude is induced in both long sides of the frame. As the^ motions of these two active sides during this, and each subsequent 90° of revolution, are in opposite direc- tions with regard to the field, the ab- solute direction of the E.M.F. induced in one side will be opposite to that in- duced in the other. However, it will be seen that both induced E.M.F. 's are in the same cyclic direction round the closed frame. The resultant sum of these components will give, therefore, a total E.M.F. and current for this and each 90° of revolution, in a definite direction, depending upon the direction of rotation relative to the polarity of the field. The maximum value of the induced E.M.F. and current is attained when the plane of the frame lies parallel to the lines of force, and will diminish steadily while remaining in the same direction round the frame until the second 90° is completed, when the total E.M.F. and current again become zero. During the next two successive 90° arcs of revolution which complete the entire revolution of 360°, the actual di- rection of rotation of the frame of course remains the same. However, the relative position of the active sides is now reversed, as is also the direction of rotation of each relative to the field. In consequence of this the direction of the resultant induced E.M.F. and cur- rent in the frame during the second half of the revolution will be opposite to that during the first half revolution considered. The changes in magnitude from zero to maximum and back to zero take place as before. The effect of continuous rotation of the frame is therefore to create surges of current in alternate directions for each revolution. The complete change from zero to the maximum in one direc- tion, back to zero, and again through a maximum in the opposite direction, back to zero, is known as a cycle. The rate of alternation is known as the frequency or periodicity, and is measured by the number of cycles per second. Suppose the form of the rectangular frame conductor to be modified, by leav- ing one short side open at the middle where the shaft crosses. Let the open ends be led out along the shaft, and each electrically connected to one of two conducting rings mounted side by side on the shaft so as to be insulated from it and from each other. We should then have a very elementary form of alter- nating current dynamo, or alternator, and by rotating the frame and u-ing collectmg brushes pressing on the rin^is, could lead an alternating current aw^y to an external circuit. Instead of two collecting ring-^ vee can arrange one ring split into two halves, each segment insulated from the other and from the shaft. By connect- ing the open ends of our rectangular conductor to these two segments, and by using a pair of brushes suitably placed, we can arrange to reverse the segments under each brush simultaneously with the reversal in the direction of current in the conductor. By this means we ob- tain an elementary direct current dy- namo, the split ring constituting the simplest form of commutator. The practical construction of an ar- mature is based upon the foregoing principle. To build up the induced
