Influence  Machines. — These  machines  differ  from  that  just  described  in
that  they  depend  upon  the  principle  of  influence  and  the  principle  of  re-
ciprocal accumulation.  Let  there  be  two  insulated  conductors  {A  and  B,
Fig.  9),  electrified  ever  so  little,  one  positively,  the  other  negatively.  Let
a  third  insulated  conductor  (C),  which  will  be  called  a  carrier,  be  arranged

*  "  Experiraenta  Nova,"  Magdeburgica.
IM  1-1    1  NCE    MACHINES.
•3


to  move  so  that  it  first  approaches  A  and  then  B,  and  so  forth.  If  touched
while  under  the  influence  of  the  small  positive  charge  on  A,  it  will  acquire
a  small  negative  charge.  Suppose  that  it  then  moves  on  and  gives  this
negative  charge  to  B.     Then  let  it  be  touched  while  under  the  influence  of


Fig.  8.
B,  so  acquiring  a  small  positive  charge.  When  it  returns  toward  A,  let
it  give  up  this  positive  charge  to  A,  thereby  increasing  its  positive  charge.
Then  A  will  act  mure  powerfully,  and  on  repeating  the  former  operations,
both  B  and  A  will  become  more  highly  charged.     Each  accumulates  the


F10.9.
charges  derived  by  influence  from  the  other.  This  is  the  fundamental
action  of  the  machines  in  question.  The  modern  influence  machine  dates
from  1S60,  when  C.  F.  Varley  produced  a  form  with  six  carriers  mounted
on  a  rotating  disk  of  glass.     This  was  followed  in   1865  by  the  machine


14


ELECTRO-PHYSICS.
of  Holt  and  that  of  Tbpler,  and  in  1867  by  those  of  Kelvin.     Then  ft
lowed  the  Wimshurst  machines,  perhaps  the  most  widely  used  of  all.
Wims/turst's  /ri/fur/nr  Machine. — We  have  in  this  machine  no  fixed  field
plates.  In  its  simplest  form  it  consists  (Fig.  10)  of  two  circular  plates  of
varnished  glass,  which  are  geared  to  rotate  in  opposite  directions.
