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 sl'aft, 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 Ting collecting brushes pressing on the rings, could lead an alternating current away to an external circuit.
Instead of two collecting ring3 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 E.M.F., insulated conductors are wound in coils about an internal coil of soft iron, which serves to concentrate the lines of force within the coils.
To avoid the loss of power and the heating effect due to eddy-currents in duced by the field in the core itself, this must be laminated, or built up on the shaft of the stampings, insulated from one another by shellac. In large ma chines, the sections are often pierced in addition, so as to form channels, and by means of distance pieces left on the shaft at intervals when assembling, ventilation and dissipation of internal heat are secured.
The simplest form of armature so constructed is the shuttle or Siemens' armature, consisting of a simple coil of many turns. This form has its prac tical limitations, the principle being the fluctuation of voltage during each revo lution when used in a direct-current ma chine, and the tendency to self-induction which increases very rapidly with the number of turns, when used in an alter nator, thus limiting the voltage capa city of the machine.