﻿Method 
  of 
  Testing 
  Photographic 
  Shutter's. 
  785 
  

  

  the 
  current 
  through 
  a 
  lamp 
  as 
  resistance 
  being 
  made 
  periodi- 
  

   cally 
  intermittent 
  by 
  means 
  of 
  a 
  wire 
  interrupter, 
  an 
  electri- 
  

   cally 
  maintained 
  tuning 
  fork, 
  or, 
  for 
  the 
  higher 
  frequencies, 
  

   a 
  microphone 
  hummer*. 
  The 
  wire 
  interrupter, 
  which 
  is 
  

   merely 
  an 
  electrically 
  maintained 
  monochord, 
  can 
  be 
  set 
  to 
  

   the 
  desired 
  frequency 
  by 
  the 
  help 
  of 
  a 
  tuning-fork 
  ; 
  the 
  

   maintained 
  fork 
  and 
  the 
  hummer 
  give 
  constant 
  and 
  known 
  

   frequencies, 
  and 
  do 
  not 
  require 
  setting. 
  The 
  pulsating 
  

   current 
  is 
  led 
  through 
  a 
  primary 
  coil 
  of 
  50 
  or 
  100 
  turns, 
  

   and 
  over 
  this 
  is 
  placed 
  a 
  movable 
  secondary 
  coil 
  connected 
  

   directly 
  to 
  the 
  vibration 
  galvanometer, 
  which 
  is 
  tuned 
  to 
  

   resonance 
  with 
  the 
  source 
  of 
  current. 
  The 
  amplitude 
  of 
  its 
  

   vibration 
  can 
  be 
  brought 
  to 
  the 
  desired 
  amount 
  by 
  changing 
  

   the 
  position 
  of 
  the 
  secondary 
  coil 
  relatively 
  to 
  the 
  primary. 
  

  

  The 
  distance 
  from 
  mirror 
  to 
  plate 
  is 
  usually 
  about 
  

   100 
  cms. 
  

  

  III. 
  Determination 
  of 
  the 
  Efficiency. 
  

  

  For 
  a 
  more 
  complete 
  test 
  the 
  efficiency, 
  in 
  addition 
  to 
  the 
  

   total 
  duration 
  of 
  exposure, 
  is 
  determined. 
  If 
  r 
  denote 
  the 
  

   total 
  duration 
  of 
  the 
  exposure, 
  T 
  the 
  equivalent 
  exposure, 
  a 
  

   the 
  area 
  of 
  the 
  shutter 
  aperture 
  at 
  the 
  instant 
  t, 
  and 
  A 
  the 
  

   maximum 
  opening, 
  we 
  have 
  the 
  relation 
  

  

  f. 
  

  

  AT=| 
  adt 
  ; 
  the 
  efficiency 
  = 
  - 
  = 
  

  

  _ 
  \ 
  adt 
  

   T 
  Jo 
  

  

  T 
  At 
  

  

  Thus 
  it 
  is 
  necessary 
  to 
  obtain 
  a 
  record 
  of 
  the 
  area 
  of 
  the 
  

   shutter 
  opening 
  at 
  every 
  instant 
  of 
  the 
  exposure. 
  

  

  For 
  the 
  determination 
  of 
  the 
  efficiency 
  the 
  method 
  employed 
  

   is 
  essentially 
  that 
  proposed 
  by 
  Sir 
  William 
  Abney, 
  who 
  uses 
  

   a 
  siren 
  to 
  measure 
  the 
  time. 
  We 
  proceed 
  as 
  follows 
  : 
  — 
  The 
  

  

  Ficr. 
  4. 
  

  

  shutter 
  is 
  removed 
  from 
  the 
  vibrating 
  beam 
  of 
  light 
  (see 
  

   fig. 
  4) 
  so 
  that 
  a 
  continuous 
  sine 
  curve 
  is 
  recorded 
  over 
  the 
  

  

  ♦ 
  Proc. 
  Roy. 
  See. 
  A. 
  vol. 
  Ixxviii. 
  p. 
  208 
  (1906). 
  

  

  