524  M.  G.  Quincke  on  Electrolysis. 
stretched,  passing  through  the  sealing-wax  cement  of  the  smaller 
sides  of  the  trough  to  mercury-cups  which  were  attached  to  the 
outside  of  the  trough.  By  means  of  a  Wheatstone's  bridge  the 
resistance  of  this  platinum  wire  was  compared  with  the  resistance 
of  a  German-silver  wire  of  almost  the  same  dimensions  and  at  a 
constant  temperature.  A  very  homogeneous  brass  wire  1000 
millims.  in  length  and  0*32  millim.  in  diameter  served  as  mea- 
suring-wire. The  current  of  a  Grove's  battery  of  seven  pairs  was 
passed  through  the  divided  circuit  as  long  as  was  necessary  for 
observing  a  deflection  on  the  reflecting  galvanometer ;  and  the 
disturbing  influence  of  thermocurrents  was  avoided  by  alter- 
nating observations  with  opposite  direction  of  the  principal  cur- 
rent of  the  battery. 
With  this  apparatus  the  resistance  P  of  the  platinum  wire 
could  be  compared  with  that  of  the  German-silver  wire,  known 
in  mercury  units,  with  accuracy  to  about  one  5000th  part. 
The  resistance  was  found  to  be 
P  =  16*207  m.  u.,  or  P  =  16*204  m.  u. 
at  14°*4  C,  according  as  the  glass  trough  was  full  of  air,  or  was 
filled  with  dilute  sulphuric  acid  of  the  specific  gravity  1*16  to  a 
height  of  49  millims.  These  numbers  may  be  regarded  as  equal, 
taking  into  account  the  difficulty  of  maintaining  at  the  same 
temperature  the  wire  and  the  dilute  sulphuric  acid. 
In  another  series  of  experiments  the  resistance  of  the  plati- 
num wire  was  determined  in  air,  then  after  the  trough  had  been 
filled  to  a  height  of  47*3  millims.  with  dilute  sulphuric  acid  of 
the  specific  gravity  1*109,  and  finally  after  the  dilute  sulphuric 
acid  had  been  removed  from  the  trough  by  means  of  a  siphon. 
These  three  determinations  at  15°*6  C.  gave 
P=16*280  m.  u.,     16-278  m.  u.,  and  16*278  m.  u. 
We  see  thus  that  the  resistance  of  platinum  wire  remains  un- 
changed whether  there  is  air  or  dilute  sulphuric  acid  in  the  glass 
trough.  The  same  result  was  obtained  when  the  resistance  was 
determined  on  first  closing  the  current  after  the  sulphuric  acid 
had  been  poured  in. 
The  reason  is,  the  immediate  polarization  of  the  platinum 
wire,  the  liberation  of  oxygen  and  hydrogen  by  the  currents 
which  branch  off  from  the  wire  into  the  liquid. 
Let  A  be  the  end  of  the  platinum  wire  where  the  electrical 
current  enters,  B  the  end  where  the  current  emerges.  After  the 
current  had  passed  for  some  time  through  the  platinum  wire, 
contact  was  broken  and  a  freshly  ignited  platinum  wire,  C,  was 
dipped  in  the  sulphuric  acid  near  the  end  A  or  B  of  the  other 
wire.  If  the  freshly  ignited  wire  and  the  horizontal  one  in  the 
glass  trough  were  then  connected  by  the  wire  of  a  delicate  gal- 
