Alternating  Current  on  a  Polarized >le  Electrode.       333 
greater  E.M.F.  alteration,  than  the  same  concentration 
increase. 
So  soon  as  the  alternating  current  ceases  to  flow,  the 
E.M.F.  also  instantaneously  ceases  to  exist.  There  is  no 
after-effect. 
The  quantitative  consideration  of  this  effect  requires  an 
hypothesis  of  the  mechanism  of  polarization,  and  is  intimately 
bound  up  with  the  nature  of  polarization  capacity.  In  an- 
tagonism to  what  he  called  the  "  Ladungsstromtheorie "  of 
Helmholtz,  which  looks  on  the  action  of  the  alternating  cur- 
rent as  the  charging  and  discharging  of  the  condenser-like 
double-layer,  Warburg  introduced  what  he  called  the 
"  Leitungsstromtheorie/'  which  disregards  the  part  played 
by  the  double-layer,  and  recognizes,  alone,  the  changes  of 
ionic  concentration  which  take  place  in  the  neighbourhood 
of  the  electrode  and  the  diffusion  which  goes  on  simul- 
taneously. These  two  points  of  view  lead  to  different  results, 
which  may  be  tested  by  experiment:  viz.,  according  to  the 
Helmholtz  view,  the  polarization-capacity  is  independent  of 
the  frequency ;  according  to  the  Warburg  view,  it  varies  as 
/t^  while  at  the  same  time  the  phase- alteration  is 
V  -e  requency  ;  L 
different  according  to  these  two  theories.  Kriiger,  taking 
into  account  both  these  elements  of  polarization  capacity,  has 
shown  that  the  one  or  the  other  predominates,  according  to 
the  nature  of  the  electrode  and  the  concentration  of  the 
electrolyte.  In  dealing,  however,  with  the  asymmetry,  it  is 
clear  that  the  Warburg  form  alone  comes  into  consideration. 
The  double-layer  can  introduce  no  asymmetry,  and,  indeed, 
the  experiments  of  Kriiger  show  that,  in  the  cases  considered, 
this  part  of  the  polarization  capacity  is  small  compared 
with  the  other. 
The  method  I  have  here  used  is  as  follows  : — An  alter- 
nating current  of  pure  sine  form  passes  through  the  electro- 
lyte. The  resulting  changes  of  concentration  follow  also  an 
harmonic  law,  and  the  asymmetry  of  the  resulting  polarization, 
calculated  by  the  Nernst  formula,  gives  an  E.M.F.  in  one 
direction,  which  causes  the  observed  direct  current.  It  may 
be  objected,  that  the  symmetry  of  the  current  and  therefore 
of  the  concentration  changes  is  disturbed  by  this  very  effect. 
The  direct  current  observed,  however^  was  always  a  small 
fraction  of  the  alternating  current,  and  the  alteration  of  the 
ion-concentration  may  relatively  without  error  be  considered 
as  due  to  the  undisturbed  alternating  current.  Of  course  a 
method  of  successive  approximation  may  be  applied;  i.  e.  from 
the  calculation  I  have  made,  a   nearer  approximation  of  the 
