Prof.  J.  C.  Maxwell  on  Electric  Induction.  537 
between  the  external  system  and  its  trail  of  images  as  expressed  in 
the  description  of  these  images  in  the  first  part  of  this  paper  (§§  6, 
7,  8,  9),  which  is  simply  an  explanation  of  the  meaning  of  equation 
(16)  combined  with  the  definition  of  P0'  in  §  24. 
Note  to  the  preceding  Paper. 
Ai  the  time  when  this  paper  was  written,  I  was  not  able  to  refer 
to  two  papers  by  Prof.  Felici,  in  Tortolini's  '  Annali  di  Scienze'  for 
1853  and  1854,  in  which  he  discusses  the  induction  of  currents  in 
solid  homogeneous  conductors  and  in  a  plane  conducting  sheet,  and 
to  two  papers  by  E.  Jochmaun  in  Crelle's  Journal  for  18G4,  and  one 
in  Posaiendorff's  'Aunalen'  for  1864,  on  the  currents  induced  in  a 
rotating  conductor  by  a  magnet. 
Neither  of  these  writers  has  attempted  to  take  into  account  the 
inductive  action  of  the  currents  on  each  other,  though  both  have 
recognized  the  existence  of  such  an  action,  and  given  equations  ex- 
pressing it.  M.  Felici  considers  the  case  of  a  magnetic  pole  placed 
almost  in  contact  with  a  rotating  disk.  E.  Jochmann  solves  the 
case  in  which  the  pole  is  at  a  finite  distance  from  the  plane  of  the 
disk.  He  has  also  drawn  the  forms  of  the  current-lines  and  of  the 
equipotential  lines,  in  the  case  of  a  single  pole,  and  in  the  case  of 
two  poles  of  opposite  name  at  equal  distances  from  the  axis  of  the 
disk,  but  on  opposite  sides  of  it,  and  has  pointed  out  why  the  cur- 
rent-lines are  not,  as  Matteucci  at  first  supposed,  perpendicular  to  the 
equipotential  lines,  which  he  traced  experimentally. 
I  am  not  aware  that  the  principle  of  images,  as  described  in  the 
paper  presented  to  the  Royal  Society,  has  been  previously  applied 
to  the  phenomena  of  induced  currents,  or  that  the  problem  of  the 
induction  of  currents  in  an  infinite  plane  sheet  has  been  solved,  taking 
into  account  the  mutual  induction  of  these  currents,  so  as  to  make 
the  solution  applicable  to  a  sheet  of  any  degree  of  conductivity. 
The  statement  in  equation  (10),  that  the  motion  of  a  magnetic 
system  does  not  produce  differences  of  potential  in  the  infinite  sheet, 
may  appear  somewhat  strange,  since  we  know  that  currents  maybe  col- 
lected by  electrodes  touching  the  sheet  at  different  points.  These 
currents,  however,  depend  entirely  on  the  inductive  action  on  the  part 
of  the  circuit  not  included  in  the  sheet ;  for  if  the  whole  circuit  lies 
in  the  plane  of  the  sheet,  but  is  so  arranged  as  not  to  interfere  with 
the  uniform  conductivity  of  the  sheet,  there  will  be  no  difference  of 
potential  in  any  part  of  the  circuit.  This  is  pointed  out  by  Felici, 
•who  shows  that  when  the  currents  are  induced  by  the  instantaneous 
magnetization  of  a  magnet,  these  currents  are  not  accompanied  by 
differences  of  potential  in  different  parts  of  the  sheet. 
"When  the  sheet  is  itself  in  motion,  it  appears,  from  art.  600 
of  my  treatise  'On  Electricity  and  Magnetism,'  that  the  electric  po- 
tential of  any  point,  as  measured  by  means  of  the  electrodes  of  a  fixed 
circuit,  is 
