﻿Earth-inductor 
  without 
  Sliding 
  Contacts. 
  917 
  

  

  certain 
  differences 
  from 
  the 
  model 
  previously 
  described, 
  which 
  

   I 
  consider 
  to 
  be 
  advantageous. 
  Hence 
  I 
  shall 
  now 
  describe 
  

   the 
  instrument 
  in 
  its 
  present 
  form 
  with 
  the 
  help 
  of 
  figs. 
  1, 
  2, 
  

   and 
  3 
  (PL 
  XXVII.). 
  

  

  The 
  conducting 
  coil 
  a 
  is 
  placed 
  so 
  as 
  to 
  turn 
  around 
  an 
  

   axis 
  y 
  in 
  a 
  wooden 
  arched- 
  frame 
  b, 
  which 
  is 
  rotatable 
  round 
  

   an 
  axis 
  x 
  normal 
  to 
  y 
  in 
  a 
  fixed 
  wooden 
  frame 
  c, 
  which 
  

   carries 
  the 
  driving 
  mechanism 
  d 
  ; 
  for 
  these 
  axes 
  bronze 
  

   ball-bearings 
  are 
  employed 
  throughout. 
  Fig. 
  1 
  shows 
  the 
  

   instrument 
  with 
  a 
  horizontal 
  axis 
  x, 
  adapted 
  for 
  induction 
  

   by 
  the 
  total 
  magnetic 
  field 
  "of 
  the 
  earth, 
  or 
  by 
  the 
  vertical 
  

   component 
  alone, 
  according 
  as 
  the 
  ^'-component 
  runs 
  along 
  

   it 
  transversely 
  or 
  longitudinally; 
  fig. 
  2 
  shows 
  the 
  same 
  with 
  

   vertical 
  axis 
  x 
  arranged 
  for 
  induction 
  by 
  the 
  earth's 
  horizontal 
  

   component 
  ; 
  fig. 
  3 
  shows 
  the 
  arrangement 
  of 
  the 
  unbroken 
  

   conducting 
  path 
  between 
  the 
  fixed 
  and 
  rotating 
  parts 
  of 
  the 
  

   circuit. 
  

  

  The 
  brass 
  shaft 
  to 
  the 
  conducting 
  coil 
  a 
  is 
  hollow 
  and 
  

   terminates 
  at 
  the 
  one 
  end 
  with 
  a 
  bevel-wheel 
  gearing 
  in 
  the 
  

   particular 
  model, 
  with 
  the 
  nearest 
  end 
  of 
  another 
  hollow 
  

   shaft 
  (which, 
  however, 
  is 
  of 
  plaited 
  bronze 
  wire 
  and 
  is 
  

   therefore 
  flexible) 
  whose 
  other 
  end 
  is 
  fixed 
  rigidly 
  in 
  the 
  

   framework 
  c. 
  At 
  each 
  rotation 
  of 
  the 
  frame 
  b 
  round 
  the 
  

   axis 
  of 
  x 
  there 
  occurs 
  therefore 
  an 
  equal 
  rotation 
  of 
  the 
  coil 
  

   a 
  round 
  the 
  axis- 
  of 
  y 
  in 
  such 
  a 
  manner 
  that 
  the 
  conducting 
  

   windings 
  'of 
  the 
  coil 
  can 
  now 
  without 
  further 
  ado 
  be 
  con- 
  

   nected 
  with 
  the 
  stationary 
  current 
  circuit 
  by 
  a 
  flexible 
  cable 
  

   led 
  through 
  the 
  two 
  hollow 
  shafts. 
  This 
  is 
  a 
  concentric 
  

   double 
  cable 
  of 
  very 
  fine 
  twisted 
  copper 
  wire 
  of 
  '0123 
  cm. 
  2 
  con- 
  

   ducting 
  cross 
  section 
  for 
  each 
  of 
  the 
  two 
  current 
  paths. 
  The 
  

   coil 
  is 
  wound 
  in 
  a 
  square 
  groove 
  of 
  2*1 
  cms. 
  side, 
  and 
  has 
  

   290 
  turns 
  (X) 
  with 
  a 
  mean 
  diameter 
  of 
  25 
  cms. 
  (D). 
  It 
  is 
  

   made 
  of 
  insulated 
  round 
  copper 
  wire 
  of 
  '0113 
  cm. 
  2 
  conducting 
  

   section 
  and 
  3*2 
  ohms 
  total 
  resistance 
  (W). 
  Accordingly 
  

   there 
  results 
  a 
  mean 
  electromotive 
  force 
  e 
  — 
  7rlO~ 
  8 
  XD 
  2 
  nF 
  

   = 
  5698 
  . 
  10~ 
  6 
  ftF 
  volts, 
  where 
  n 
  denotes 
  the 
  number 
  of 
  rotations 
  

   per 
  second 
  and 
  F 
  the 
  strength 
  of 
  the 
  magnetic 
  field 
  — 
  which 
  

   must 
  be 
  taken 
  as 
  the 
  total, 
  vertical, 
  or 
  horizontal, 
  component 
  

   according 
  to 
  the 
  experimental 
  arrangement. 
  

  

  The 
  constructed 
  model 
  allows, 
  when 
  worked 
  by 
  hand, 
  the 
  

   value 
  n 
  = 
  5 
  to 
  be 
  attained 
  with 
  which 
  a 
  throttling 
  by 
  the 
  self- 
  

   inductance 
  of 
  the 
  moderately 
  wide 
  wound 
  induction-coil 
  

   attains 
  no 
  appreciable 
  value. 
  The 
  distance 
  of 
  the 
  windings 
  

   being 
  *15 
  cm. 
  (d), 
  the 
  self-inductance, 
  

  

  L„= 
  2ttDN 
  2 
  (log 
  nat 
  id- 
  1 
  . 
  D 
  - 
  2), 
  

   is 
  about 
  858 
  kilom. 
  (or 
  350 
  -1 
  ^), 
  so 
  that 
  the 
  impedance 
  

  

  