After the coil has rotated a half revolution, conductor DC begins to move downwards and AB upwards. The polarity induced is now in reverse to that for the 1 st half revolution, D being + ve relative to C and B is + ve relative to A. Terminal X is now the + ve terminal and Y is the negative. An alternating E.M.F. is generated, as shown in the diagram (Fig 91) which also illustrates four positions of the coil viewed from the Slip-ringend. For position 1, A and D are moving horizontally along the field and no E.M.F. is being generated. A similar condition exists for position 3, but for position 2 and 4 maximum E.M.F. is being generated, since field-cutting at right-angles is taking place. For intermediate position, the general condition of E.M.F. generation, as represented by E Blv sin0 , is followed, since the conductors are cutting a uniform field at an angle but are moving at a uniform velocity. Thus the E.M.F. generated at any instant is not constant but varies and it is customary to use a small letter for what is termed the instantaneous value.