422  Prof.  Challis  on  the  Hydro  dynamical 
tary  experiment  above  described.  This  point  is  left  in  ambi- 
guity by  the  experiments,  as  not  admitting  probably  of  being 
decided  except  by  the  combination  of  experiment  with  a  true 
theory.  The  present  theory  furnishes  for  deciding  it  the  fol- 
lowing considerations. 
38.  In  the  description  of  the  fundamental  experiment  Faraday 
states  (referring  to  fig.  16  in  plate  iii.  p.  131,  of  the  Phil. 
Trans,  for  1832)  that  "when  the  galvanometer-needle  was  de- 
flected, its  north  or  marked  end  passed  eastward,  indicating  that 
the  wire  A  received  negative  and  the  wire  B  positive  electri- 
city." As  the  wire  A  belongs  to  the  terminal  applied  to  the 
eastern  edge  of  the  plate  and  the  wire  B  to  that  applied  to  the 
western  edge  (the  two  wires  being  in  fact  a  single  wire  consti- 
tuting the  circuit),  I  was  led  by  the  above  statement  to  suppose 
that  the  current  proceeded  out  of  the  plate  on  the  west  side  and 
entered  it  on  the  east  side  (Principles  of  Physics,  p.  636). 
But  on  the  assumption  that  the  spiral  motion  of  the  galvanic 
current  has  been  correctly  determined  by  the  foregoing  argu- 
ment to  be  dextrorsum,  the  truth  of  that  supposition  can  be 
tested  as  follows  by  the  observed  direction  of  the  displacement 
of  the  galvanometer-needle.  The  wire  proceeding  from  the 
western  edge  was  made  to  pass  beneath  the  needle  in  the  direc- 
tion from  south  to  north.  Hence  the  circular  motion  about  the 
rheophore  would  conspire  on  the  west  side  of  the  north  end  of 
the  needle  with  the  entering  magnetic  streams  resolved  perpen- 
dicularly to  the  axis  of  the  needle,  and  be  opposed  to  them  on 
the  east  side.  Thus  there  would  be  an  excess  of  pressure  on  the 
east  side,  and  the  deflection  would  be  westward.  But  Faraday 
says  that  the  marked  end  "passed  eastward."  It  must  there- 
fore be  concluded,  in  order  to  reconcile  the  theory  with  experi- 
ment, that  the  direction  of  the  current  is  the  reverse  of  that  as- 
sumed, proceeding  out  of  the  east  side  and  entering  at  the  west 
side.  It  is  true  that  the  fact  and  theory  would  agree  if  the 
spiral  motion  might  be  assumed  to  be  sinistrorsum ;  but  the 
previous  argument  is  opposed  to  this  supposition.  There  re- 
mains, therefore,  only  the  inference  that  the  current  actually 
flows  along  the  rheophore  from  the  east  to  the  west  side  of  the 
plate.  Moreover,  as  will  presently  be  shown,  the  phenomena 
of  the  mutual  action  between  a  magnet  and  a  mass  of  copper 
relatively  in  motion  are  explainable  by  the  theory  only  in  case 
the  current  has  this  direction,  which  will  at  least  give  evidence 
of  the  consistency  of  the  theory  with  itself. 
39.  Let  us  take  the  case  of  a  circular  plate  of  copper  placed 
under  a  magnetic  needle  and  caused  to  rotate  about  an  axis  co- 
incident with  that  of  the  magnet,  and  let  the  direction  of  rota- 
tion be  from  west  through  north  to  east,  or  that  of  the  move- 
