This  may  be  done  by  including  in  its  circuit  a  current-
galvanometer,  and  determining  the  current  7  through
the  wire  in  amperes,*  when  Vis  read  off  in  volts  *  on  the
potential  instrument.  The  resistance  of  the  wire  with
that  of  the  current-galvanometer  is  in  ohms  V/<y,  and
this  is  to  be  used  as  the  value  of  R  in  equation  (72).
If  a  galvanometer  of  high  resistance  be  not  available,
an  approximate  test  can  be  made  by  means  of  a  sensitive
galvanometer  of  low  resistance.  The  battery  and  gal-
vanometer are  joined  in  series  with  a  resistance  R,  and
again  with  a  resistance  R'.  Let  D  and  Dr  be  the  de-
flections, which  must  have  a  difference  comparable  with
either.  Then,  supposing  E  and  r  to  be  the  same  in
both  cases,  and  putting  Gr  for  the  resistance  of  the
galvanometer  we  have

E  E

D  =  m  ^  -  ~  -  y      1)  =  m--u  -  ~  -  >
R  +  G  +  r  R'  +  Cr  -f  r

where  m  is  a  constant.
Therefore  we  find


,_=_<,.    .    .    .    (73)

Mance  has  shown  how  to  determine  the  resistance  of    Mance's
a   battery   by   means   of    Wheatstone's   Bridge.     The     ^f^he
battery  is  placed  in  the  position  BD  of  Fig.  70  above,  Resistance
and  a  key  is  connected  between  B  and  C.     The  resist-
*  See  Vol.  II.,  also  the  Note  in  the  Appendix  to  the  present  volume.
416  COMPARISON  OF  RESISTANCES.
