190     On  Testing  the  Metal-resistance  of  Telegraph-wires. 
the  wires,  the  end  of  the  index,  and  the  radial  line  on  the  bottom 
of  the  frame  must  coincide,  so  that  parallax  is  avoided.  The 
object  of  the  frame  is  therefore  twofold  :  first,  it  serves  to  limit 
the  vibration  of  the  needle ;  and,  secondly,  it  gives  us  a  very- 
exact  means  of  noting  a  deflection.  There  is  a  small  needle- 
point attached  to  the  bottom  plate,  forming  a  prolongation  of 
the  line ;  and,  finally,  orpon  the  dial-plate  are  engraved,  instead  of 
a  number  of  degrees  or  other  arbitrary  divisions,  simply  a  line 
at  right  angles  to  the  coil,  to  serve  as  a  zero-line,  and  two 
radial  lines  on  each  side,  the  use  of  which  we  shall  presently  see. 
There  is  a  small  socket  in  the  top  of  the  cover  of  the  instru- 
ment, immediately  over  the  centre  of  suspension  of  the  needle,  in 
which  may  be  fixed  a  rod  carrying  a  directing  magnet.  In 
testing  ordinary  resistance,  this  magnet  may  be  made  to  oppose 
the  earth's  magnetism,  so  as  to  render  the  needle  nearly  astatic; 
or  it  may  be  made  to  counteract  a  deflection,  as  in  taking  battery 
or  line-resistance,  as  already  explained,  so  as  to  bring  the  needle 
into  its  most  sensitive  position ;  or  it  may  be  removed  altogether. 
Besides  all  the  ordinary  tests,  to  which  a  differential  galva- 
nometer can  be  applied,  we  are  able  with  this  instrument  to 
measure  approximately  electromotive  force ;  and  for  this  purpose 
the  radial  divisions  above  referred  to  are  engraved  on  the  dial-plate. 
The  method  is  that  described  by  Wheatstone  in  the  Philoso- 
phical Transactions  in  1843.  There  are,  I  should  have  said 
before,  three  shunts  to  each  coil,  cutting  off  respectively  T%, 
T9^5,  and  j9o^39o  of  the  current. 
In  order  to  determine  the  position  of  the  two  divisions  on  the 
left-hand  side,  I  insert  the  f^  shunt,  on  each  coil;  a  good  ordi- 
nary DaniePs  cell,  such  as  is  commonly  used  on  the  line,  is  then 
taken  and  joined  up  through  the  galvanometer-  and  resistance- 
coils,  as  shown  in  fig.  4.  Resistance  is  then  inserted  until  a 
convenient  deflection  is  obtained;  the  frame  is  then  made  to 
coincide  with  the  index,  and  the  position  of  the  needle-point 
marked  on  the  dial-plate ;  10  units  are  then  added  to  the  re- 
sistance, the  deflection  is  of  course  reduced,  and  the  frame  is 
made  to  coincide  with  the  index  a  second  time,  and  the  position 
of  the  needle-point  again  marked  on  the  dial.  The  two  points 
marked  have  then  conspicuous  radial  lines  engraved  through 
them.  On  the  right-hand  side  we  do  exactly  the  same,  except 
that  the  unit-cell  is  formed  of  an  amalgamated  zinc  plate  in  1 
of  sulphuric  acid  to  12  of  water,  and  a  copper  plate  in  a  satu- 
rated solution  of  nitrate  of  copper;  the  electromotive  force  of 
this  element  being  about  1  volt. 
"When  the  electromotive  force  of  a  cell  or  battery  has  to  be 
measured,  we  join  it  up  through  the  galvanometer-  and  resist- 
ance-coils, making  the  needle  deflect  either  to  the  right  or  left, 
