﻿506 
  Dr. 
  W. 
  Mansergh 
  Varley 
  on 
  Magnetism 
  induced 
  

  

  h 
  ] 
  — 
  Vi\ 
  whence 
  K 
  ' 
  = 
  e' 
  & 
  ; 
  and 
  similarly 
  for 
  the 
  second 
  

   system 
  H 
  " 
  = 
  c 
  n 
  '6 
  ;/ 
  , 
  whence 
  

  

  So" 
  

  

  1 
  8' 
  

  

  8"' 
  

  

  H/, 
  H 
  '' 
  being 
  the 
  fields 
  in 
  the 
  cores 
  of 
  the 
  solenoids 
  in 
  the 
  

  

  two 
  cases, 
  £/, 
  8 
  " 
  the 
  deflexions 
  due 
  to 
  the 
  currents 
  alone^ 
  

  

  and 
  a\ 
  b 
  f 
  , 
  c', 
  c'\ 
  and 
  c 
  constants. 
  

  

  A 
  certain 
  specimen 
  of 
  iron 
  was 
  placed 
  in 
  each 
  of 
  the 
  two 
  

  

  solenoids 
  in 
  turn 
  and 
  magnetized 
  by 
  direct 
  current 
  to 
  the 
  

  

  same 
  intensity 
  in 
  the 
  two 
  cases, 
  all 
  deflexions 
  being 
  noticed. 
  

  

  Now 
  we 
  have 
  H^H"; 
  and 
  the 
  relation 
  of 
  the 
  current 
  

  

  8' 
  

   deflexions 
  w/~ 
  c 
  gives 
  the 
  reduction 
  factor, 
  by 
  means 
  of 
  

  

  which 
  it 
  is 
  possible 
  to 
  reduce 
  the 
  deflexions 
  obtained 
  with 
  

   the 
  second 
  system 
  of 
  coils 
  to 
  the 
  same 
  unit 
  as 
  those 
  obtained 
  

   with 
  the 
  first 
  system. 
  All 
  measurements 
  were 
  reduced 
  to 
  

   those 
  of 
  the 
  645 
  turn 
  system, 
  the 
  reduction 
  factor 
  by 
  which 
  

   the 
  current 
  deflexion 
  in 
  the 
  case 
  of 
  the 
  140 
  turn 
  system 
  had 
  

   to 
  be 
  multiplied 
  being 
  found 
  as 
  a 
  mean 
  of 
  12 
  measurements 
  

   to 
  be 
  1-46. 
  

  

  6. 
  The 
  capacities 
  of 
  the 
  leyd 
  en-jars 
  used 
  were 
  determined 
  

   by 
  comparison 
  with 
  a 
  standard 
  high 
  resistance 
  by 
  the 
  tuning- 
  

   fork 
  method, 
  modified 
  so 
  as 
  to 
  be 
  suitable 
  for 
  measuring 
  

   accurately 
  capacities 
  as 
  low 
  as 
  1000 
  cms. 
  The 
  capacities 
  of 
  

   the 
  several 
  jars 
  used 
  were 
  found 
  to 
  be 
  1640, 
  2430, 
  2270 
  cms. 
  

   &c. 
  These 
  measurements 
  were 
  repeated 
  after 
  an 
  interval 
  of 
  

   several 
  months, 
  and 
  identical 
  values 
  obtained. 
  

  

  The 
  coils 
  used 
  as 
  standard 
  or 
  known 
  self-inductions 
  had 
  

   the 
  dimensions 
  given 
  in 
  the 
  following 
  table, 
  the 
  self-induc- 
  

   tions 
  being 
  calculated 
  from 
  Maxwell's 
  formula 
  *, 
  or 
  rather 
  

   Stefan's 
  formf 
  of 
  the 
  same. 
  

  

  Table 
  II. 
  

  

  Coil. 
  

  

  Mean 
  radius, 
  

  

  Breadth, 
  

  

  Thickness, 
  

  

  No. 
  of 
  turns. 
  

  

  Self-induction, 
  

  

  cms. 
  

  

  cms. 
  

  

  cms. 
  

  

  

  cms. 
  

  

  I. 
  ... 
  

  

  119 
  

  

  1-8 
  

  

  20 
  

  

  140 
  

  

  80x10° 
  

  

  II. 
  ... 
  

  

  120 
  

  

  3-5 
  

  

  2-0 
  

  

  250 
  

  

  22xl0 
  6 
  

  

  III. 
  ... 
  

  

  10-85 
  

  

  23 
  

  

  17 
  

  

  47 
  

  

  0-77x10° 
  

  

  * 
  C. 
  Maxwell, 
  Phil. 
  Trans, 
  civ. 
  p. 
  508 
  (1865) 
  ; 
  see 
  also 
  Lord 
  Kayleig-h, 
  

   Proc. 
  Roy. 
  Soc. 
  xxxii. 
  p. 
  117 
  (1881). 
  

  

  f 
  J. 
  Stefan, 
  Wied. 
  Ann. 
  xxii. 
  p. 
  112 
  (1884). 
  

  

  