350     M.  F.  Zollner  on  the  Origin  of  the  Earth's  Magnetism, 
flowing  liquid,  so  that  the  platinum  plates  which  are  immersed 
in  the  liquid  for  the  observation  of  the  current  act  in  such  a 
manner  that  the  plate  first  touched  corresponds  to  the  zinc  end, 
the  plate  which  is  last  touched  to  the  platinum  end  of  a  Grove's 
battery.  The  electromotive  force  of  these  currents  is  very  large. 
Using,  for  instance,  a  diaphragm  of  common  quartz-sand,  the 
electromotive  force  is  6*2  times  as  great  as  that  of  a  Daniell's 
cell ;  so  that  Quincke,  at  the  end  of  his  paper,  formed  the  idea 
of  making  such  a  source  of  electricity  practically  useful.  He 
remarks  as  follows  : — 
"  I  have  lastly  tried  to  make  use  of  the  water- works  of  this 
town  to  produce  an  electric  current,  as  the  large  electromotive 
force  made  me  hope  to  obtain  currents  of  practical  application/' 
The  experiments  made  with  an  instrument  constructed  for 
this  purpose  under  a  pressure  of  about  2  J  atmospheres  and  an 
hourly  use  of  5  cubic  feet  of  water,  showed  only  weak  electric 
currents.  Quincke  explains  this  result  partly  by  the  salt  con- 
tained in  the  water,  and  partly  by  oxide  of  iron  present  in  the 
pipes. 
The  electromotive  forces  seem  to  be  (according  to  the  experi- 
ments hitherto  made)  independent  of  the  thickness  and  surface 
of  the  porous  walls,  but  proportional  to  the  difference  of  pressure 
causing  the  flow  of  the  liquid. 
The  origin  of  this  source  of  electricity  has,  until  now,  not 
been  reduced  to  the  known  disturbances  of  the  electrical  equi- 
librium. 
Even  not  considering  the  possible  connexions  of  this  kind  of 
current  with  the  following  phenomena,  it  is  of  the  highest  im- 
portance for  the  present  considerations  that  in  a  common  river 
these  electric  currents  can  be  shown  to  exist  if  the  two  plates  at 
the  ends  of  galvanometer-wires  are  inserted  one  in  the  more 
quickly  flowing  middle  of  the  river,  the  other  near  its  borders*. 
It  is,  as  already  stated,  quite  immaterial  what  we  assume  to 
be  the  cause  of  these  electric  currents ;  it  is  sufficient  for  us  that 
we  can  show  their  existence  directly  by  observation.  It  is  clear 
that  these  currents  will  still  be  generated  if,  instead  of  the  metal 
plates  of  the  above  experiments,  two  rocks  projecting  into  the 
current  are  in  conducting  communication  by  means  of  the  earth. 
*  Adie  (Phil.  Mag.  vol.  xxxi.  p.  353,  1847).  "Two  slips  of  zinc  cut 
side  by  side  from  the  same  sheet  were  placed  in  a  running  brook,  the  one 
opposed  to  a  rapid  part  of  the  current,  the  other  in  a  still  place  at  the  edge. 
Connecting  these  in  usual  manner  with  the  galvanometer,  there  was  a  per- 
manent deflection  of  25°;  and  on  changing  the  respective  places  of  the  plates 
in  the  stream  without  disturbing  their  attachments  to  the  galvanometer, 
the  needles  immediately  passed  to  the  opposite  side  of  the  card ;  in  both 
cases  the  piece  of  zinc  in  the  current  acted  as  a  negative  orplatinode  plate. 
