Telegraph-wires  or  Cables  influenced  by  Earth-currents.     189 
deflection  is  either  observed,  or  (what  is  better)  counteracted  by 
a  directing  magnet,  and  the  index  brought  to  zero;  the  key  is 
then  raised;  the  current  circulates  in  both  coils  of  the  galvano- 
meter, and  any  resistance  that  may  be  opposed  to  it  in  the  resist- 
ance-coils D.  If  the  position  of  the  needle  remains  unaltered, 
the  resistance  in  the  coils  is  equal  to  the  resistance  of  the  line. 
If  it  is  necessary  to  use  shunts,  the  result  is  the  same,  provided 
the  coils  are  shunted  by  equal  shunts;  if,  however,  the  Hue 
offers  too  great  a  resistance  to  be  measured  by  the  resistance- 
coils,  we  may  shunt  a  greater  portion  of  the  current  from  the 
coil  B  of  the  galvanometer  than  from  the  coil  A.  Say,  for  in- 
stance, we  shunt  ^  of  the  current  from  A  and  -f^  from  B;  then 
the  line-resistance  will  equal  the  resistance  in  the  coil  multiplied 
by  10.  If  the  earth-current  is  not  sufficient  to  give  a  satisfac- 
tory deflection,  we  have  merely  to  insert  a  battery  of  known  re- 
sistance between  the  line  and  galvanometer,  and  subtract  its 
resistance  from  the  result.  It  is  generally  indeed  better  to  do 
so,  as  in  that  case  the  variations  in  the  earth-current  will  not  be 
so  perceptible. 
The  differential  galvanometer  I  have  designed  and  constructed 
for  the  use  of  my  inspectors  is  well  adapted  to  this  test ;  and  as 
it  has  proved  so  useful  in  practice,  I  venture  to  hope  a  short  de- 
scription of  it  may  not  be  unacceptable  to  the  readers  of  this 
Magazine. 
The  exterior  form  of  the  instrument  is  somewhat  similar  to 
Yarley's  original  universal  galvanometer,  and  is  shown  in  plan 
in  fig.  3,  with  the  connexions  in  dotted  lines. 
The  two  wires  of  the  coil  are  wound  simultaneously  and  as 
carefully  parallel  as  possible,  so  that  their  magnetic  effects  may 
be  equal.  Their  resistances  are  also  made  equal.  The  needle  is 
suspended  by  a  short  fibre  of  silk  from  a  small  bracket  in  the  in- 
terior of  the  coil,  which  bracket  slides  in  and  out  in  a  groove. 
AVhen  it  is  desired  to  render  the  instrument  portable,  we  can, 
by  turning  a  milled-head  on  top  of  the  cover,  raise  the  needle 
and  press  it  firmly  against  two  pieces  of  cork  on  the  top  of  the 
interior  of  the  coil. 
The  index  is  fixed  at  right  angles  to  the  needle.  The  end  of 
the  index  plays  between  the  two  upright  sides  of  a  small  frame. 
This  frame  is  attached  to  an  arm  which  runs  under  the  base  of 
the  instrument,  and  is  moveable  in  a  segment  of  a  circle  concen- 
tric with  the  point  of  suspension  of  the  needle.  The  bottom  of 
the  frame  is  a  small  brass  plate  with  a  line  drawn  across  it  so  as 
to  be  radial  to  the  circle  in  which  the  frame  moves.  The  top  of 
the  frame  carries  two  cross  wires,  which  intersect  each  other  in  a 
point  exactly  over  the  radial  line  on  the  bottom.  "When  a  read- 
iug  is  taken,  one  eye  only  must  be  used,  and  the  intersection  of 
