Theory  of  Pliasemet 
ers. 
89 
moving  coil  a  pointer  P  is  attached,  which  reads  on  a  fixed 
scale  of  degrees.  For  any  given  values  o£  the  currents  in 
the  fixed  coils,  the  space  distribution  of  induction-density  in 
the  gap  tends  to  vary  rapidly  at  the  slots  and  to  be  nearly 
uniform  in  the  space  between  one  slot  and  the  next.  To 
overcome  this  effect,  which  was  foreseen,  successive  stampings 
had  been  slightly  sheared  circumferentiallv,  so  that  the  line 
of  centres  for  any  slot,  instead  o£  being  quite  parallel  to  the 
axis,  was  slightly  spiral  in  reference  to  it.  The  stepped 
nature  of  the  curve  connecting  x  and  0  shows  that  the  effect 
in  question  was  only  partially  neutralized.     But  this  tends 
Fig.  3. 
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to  make  the  correspondence  of  the  direct-current  and  alter- 
nating-current tests  the  more  convincing.  It  will  be  noted 
that  the  angular  period  of  the  steps  in  the  curve  is  30°, 
the  same  as  the  angular  interval  between  successive  slots. 
The  results  are  all  set  forth  in  nV. 
The  direct  current 
