N.B. — After  using  a  battery
the  zincs  must  be  washed  and
dried,    the    porous    cells    must
be  carefully  washed,  and  com-
^■'"'i'Jlir    pletely    immersed    in    a    large
Yi  ;        -]'-n   It  quantity    of    water,    frequently

renewed.
4.  Grove's  Cell  (fig.  81)  consists  of  an  outer  glazed  earthenware,
glass,  or  ebonite  jar,  containing  amalgamated  zinc  and  10  p.c.
sulphuric  acid.  In  the  inner  porous  cell  is  placed  platinum  foil
witii  strong  nitric  acid.  The  ])lafcinum  is  the  +  positive  pole  or
anode,  the  zinc  the  -  negative  pole  or  cathode.  For  physiological
purposes,  the  small  Grove's  cells,  about  7  cm.  in  diameter  and  5  cm.
in  height,  are  very  convenient.     When  in  use  the  battery  ought  to


XXV.]


GALVANIC   BATTERIES   AND   GALVANOSCOPE.
159


be  placed  in  a  draught  chaml  er  to   prevent   the    nitrous   fumes
from  affecting  the  experimenter.
5.  Bichromate  Cell  (fig.  82).— This
consists  of  a  glass  bottle  containing
one  zinc  and  tw)  carbon  plates  im-
mersed in  the  following  mixture  : —
Dissolve  I  part  of  potassic  bichromate
in  8  parts  of  water,  and  add  i  part  of
sulphuric  acid.  The  zinc  is  attached
to  a  rod,  which  can  be  raised  when  it
is  desired  to  stop  the  action  of  the
battery.  This  cell  is  convenient
enough  when  it  is  not  necessary  to
use  a  current  of  perfectly  constant  in-
tensity.
6.  Leclanche  Cell. — The  positive
plate  is  zinc  in  ammonium  chloride
solution  (Zinc  -  pole).  The  negative
plate  is  carbon  with  manganese
dioxide  in  the  same  solution  (Car-
bon +  pole).
Other  forms  of  batteries  are
used,  but  the  foregoing  are  suffi-
cient for  the  purposes  of  these
exercises.
7.  The  Galvanoscope  or  De-
tector.
