﻿Manometer 
  for 
  H'ujli-Pressure 
  Jfixjylosions. 
  463 
  

  

  system, 
  W 
  the 
  weight 
  of 
  the 
  moving 
  parts, 
  the 
  time-period 
  

   willbe=27r, 
  /W 
  

  

  Kg 
  

  

  We 
  have 
  therefore 
  two 
  variables 
  at 
  our 
  disposal, 
  namely, 
  

   the 
  weight 
  of 
  the 
  moving 
  parts 
  and 
  the 
  controlling 
  force. 
  

   The 
  former 
  must 
  be 
  made 
  a 
  minimum, 
  the 
  latter 
  a 
  maximum. 
  

  

  In 
  most 
  instruments 
  where 
  a 
  short 
  period 
  is 
  desirable 
  the 
  

   strains 
  to 
  which 
  the 
  parts 
  are 
  subjected 
  are 
  very 
  small, 
  and 
  

   the 
  desired 
  result 
  is 
  obtained 
  by 
  decreasing 
  the 
  size 
  of 
  all 
  

   moving 
  parts, 
  and 
  using, 
  wherever 
  possible, 
  materials 
  of 
  low 
  

   density. 
  This 
  method 
  is 
  employed 
  in 
  the 
  case 
  of 
  all 
  oscillo- 
  

   graphs, 
  telegraph-recorders, 
  phonograph-receivers, 
  galvano- 
  

   meters, 
  &c. 
  

  

  In 
  the 
  present 
  ease, 
  the 
  instrument 
  having 
  to 
  withstand 
  

   pressures 
  of 
  ten 
  or 
  twenty 
  thousand 
  pounds 
  per 
  square 
  inch 
  

   applied 
  with 
  extreme 
  suddenness, 
  strength 
  becomes 
  a 
  con- 
  

   dition 
  of 
  vital 
  importance 
  ; 
  and 
  steel 
  is 
  the 
  only 
  material 
  

   which 
  will 
  withstand 
  the 
  strain. 
  We 
  cannot 
  therefore 
  use 
  

   materials 
  of 
  small 
  density, 
  neither 
  can 
  we 
  reduce 
  the 
  dimen- 
  

   sions 
  of 
  the 
  moving 
  parts 
  below 
  a 
  certain 
  limit. 
  

  

  It 
  is 
  thus 
  evident 
  that 
  we 
  must 
  have 
  recourse 
  to 
  the 
  second 
  

   variable 
  factor 
  to 
  secure 
  the 
  short 
  time-period 
  which 
  is 
  

   necessary. 
  As 
  we 
  have 
  seen 
  above, 
  the 
  controlling 
  force 
  

   brought 
  into 
  play 
  per 
  unit 
  length 
  of 
  motion 
  must 
  be 
  as 
  great 
  

   as 
  possible. 
  In 
  other 
  words, 
  we 
  must 
  use 
  the 
  stiffest 
  spring 
  

   we 
  can 
  obtain. 
  

  

  The 
  stiffness 
  of 
  a 
  spring 
  will 
  vary 
  with 
  the 
  material 
  of 
  

   which 
  it 
  is 
  made 
  and 
  its 
  shape, 
  increasing 
  for 
  a 
  given 
  size 
  

   as 
  the 
  shape 
  approaches 
  more 
  nearly 
  to 
  that 
  of 
  a 
  solid 
  bar 
  

   subjected 
  to 
  longitudinal 
  strain. 
  This 
  bar 
  can 
  be 
  made 
  as 
  

   short 
  as 
  may 
  be 
  desired, 
  and 
  in 
  theory 
  the 
  time-period 
  of 
  the 
  

   system 
  is 
  only 
  limited 
  by 
  the 
  density 
  of 
  the 
  material 
  and 
  by 
  

   its 
  modulus 
  of 
  elasticity. 
  

  

  Design 
  of 
  the 
  Recorder. 
  

  

  In 
  practice, 
  however, 
  the 
  travel 
  of 
  the 
  moving 
  parts 
  cannot 
  

   be 
  indefinitely 
  decreased, 
  for 
  the 
  deflexions 
  must 
  remain 
  of 
  

   such 
  dimensions 
  as 
  to 
  be 
  accurately 
  measurable. 
  The 
  fol- 
  

   lowing 
  figure 
  is 
  a 
  theoretical 
  representation 
  of 
  a 
  recorder 
  

   designed 
  on 
  the 
  principles 
  we 
  have 
  just 
  established. 
  

  

  A 
  cylindrical 
  groove 
  is 
  cut 
  half 
  through 
  the 
  walls 
  of 
  the 
  

   inclosure. 
  The 
  upper 
  part 
  P 
  of 
  the 
  cylinder 
  thus 
  obtained 
  

   represents 
  the 
  piston 
  of 
  our 
  indicator, 
  and 
  the 
  lower 
  portion 
  

   S 
  the 
  spring. 
  Under 
  the 
  pressure 
  of 
  the 
  explosion 
  the 
  

   piston 
  P 
  will 
  be 
  forced 
  outwards 
  a 
  certain 
  small 
  amount 
  

  

  212 
  

  

  