All  properly  constructed  resistance  coils  are,  as  has  been
stated,  made  of  wires  which  have  been  first  doubled  on
themselves  and  then   wound  double  on  their  bobbins,
and  have  therefore  no  self-induction.     The  wire  tested,
however,  and  the  connections  of  the  bridge  have  gene-
rally more  or  less  self-induction,  the  effect  of  which,
unless  the  contacts  were  made  as  described  above,  might
be  mistaken  for  those  of  unbalanced  resistance.     This


MODE  OF  OPERATING  IN  BRIDGE  METHOD.  337

mode  of  winding  the  coils  also  avoids  direct  electro-
magnetic effects  on  the  coils  on  the  galvanometer  needle
when  the  coils  are  placed  near  it.
If  on  depressing  the  galvanometer  key  at  first  as  Avoidance
described  above  a  current  is  found  to  be  produced  by  °*"P^S°~
thermoelectric  or  leakage  disturbance,  and  the  spot  of   disturb-
light  is  therefore  displaced,  the  operator  keeping  down
the  galvanometer  key  depresses  the  battery  key,  and
observes  if  there  is  any  permanent  deflection  of  the  spot
of  light  from  its  displaced  position  during  the  time  that
the  battery  key  is  kept  down.     This  is  easily  distin-
guished from  the  sudden  deflection  due  to  self-induction,
as  that  immediately  dies  away  to  zero  as  the  current
rises  to  its  permanent  value.
If  the  sudden  deflection  of  the  galvanometer,  as  it
may  be  in  the  case  of  a  dynamo  or  electro -magnet,  is
too  violent  and  long  continued,  the  reversing  key  of  the
battery  should  be  used,  the  battery  contact  made  first  in
each  case,  and  the  mean  of  the  results  taken.
