The  two  junctions  of  the  thermo-couple  are  situated  quite  close  together  so  that  they
partake  equally  in  any  changes  of  atmospheric  temperature,  but  the  "  cold  "  or
comparison  junction  is  screened  from  the  radiation  focused  by  the  mirror  M.  To
prevent  over-heating  the  thermo-couple,  when  the  telescope  is  sighted  on  a  very  hot
body,  a  diaphragm  D  is  provided,  which  can  be  swung  over  the  mouth  of  the
telescope,  thus  reducing  the  effective  aperture  and  consequently  the  radiant  heat
falling  on  the  mirror.
444


GAS  PRODUCERS


To  guide  the  pointing  of  the  telescope  an  eyepiece  E  is  provided  at  the  rear  and
through  which  can  be  seen  a  reflected  image  of  the  hot  body.  The  focusing  is  done
by  means  of  a  milled  head  H  at  the  side  of  the  telescope  and  its  accuracy  verified  by
obsening  the  reflected  image  of  the  hot  body.
In  the  indicating  outfit  the  telescope  ifi  mounted  on  a  collapsible  tripod.  To  enable
temperature  readings  to  be  made  the  thermo-couple  circuit  is  completed  through  a
sliort  length  of  flexible  cable  and  an  indicator.  This  indicator  is  an  accurately
calibrated  millivoltmeter,  which  is  either  automatically  self-leveUng  or  does  not
require  any  leveUng.
It  is  caUbrated  to  read  temperature  directly  upon  two  scales,  one  from  1000  to
2400°  F.,  the  second  from  1800  to  3600°  F.  Centigrade  scales  can  also  be  provided.
There  is  a  further  calibration  in  millivolts  by  means  of  which  the  sensibility  of  the
indicator  can  be  checked  when  desired.  The  whole  outfit  is  arranged  to  fold  up  and
drop  in  a  box,  and  is  easily  carried  by  one  man  as  it  weighs  only  about  30  lbs.
One  of  the  illustrations  shows  an  example  of  the  Centigrade  scale  of  the  indicator
suppUed  with  the  portable  outfit.
