﻿102 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1943 
  

  

  INSTRUMENTS 
  

  

  Within 
  the 
  past 
  quarter 
  century, 
  so 
  much 
  progress 
  has 
  been 
  made 
  

   in 
  the 
  commercial 
  use 
  of 
  aluminum 
  products, 
  and 
  also 
  of 
  high-vacuum 
  

   technique, 
  that 
  it 
  is 
  possible 
  for 
  one 
  who 
  is 
  familiar 
  with 
  the 
  astrono- 
  

   mers' 
  solution 
  of 
  their 
  problem 
  of 
  following 
  the 
  heavenly 
  bodies 
  with 
  

   telescopes 
  to 
  design 
  types 
  of 
  apparatus 
  for 
  utilizing 
  solar 
  radiation 
  

   for 
  power, 
  combining 
  minimum 
  expense 
  with 
  maximum 
  efficiency. 
  

  

  Formerly 
  the 
  choice 
  for 
  solar 
  mirrors 
  lay 
  between 
  mirror 
  glass 
  and 
  

   metals 
  of 
  rather 
  low 
  reflecting 
  power 
  and 
  short 
  reflecting 
  life. 
  Such 
  

   materials 
  for 
  mirrors 
  were 
  heavy, 
  costly, 
  inefficient, 
  and 
  quickly 
  

   deteriorating. 
  We 
  can 
  now 
  purchase 
  commercially 
  the 
  bright 
  reflect- 
  

   ing 
  product 
  called 
  Alcoa 
  in 
  thin 
  sheets 
  of 
  large 
  size. 
  It 
  is 
  a 
  special 
  

   preparation 
  of 
  aluminum, 
  long 
  retaining 
  its 
  reflectivity, 
  and 
  with 
  a 
  

   coefficient 
  of 
  reflection 
  for 
  solar 
  rays 
  slightly 
  exceeding 
  80 
  percent. 
  

   Frames 
  of 
  suitable 
  curvature 
  being 
  made 
  from 
  duralumin 
  shapes, 
  

   these 
  may 
  be 
  covered 
  with 
  the 
  thin 
  Alcoa 
  sheets 
  to 
  make 
  up 
  cheap, 
  

   light, 
  and 
  fairly 
  permanent 
  solar 
  mirrors. 
  

  

  In 
  consideration 
  of 
  best 
  design, 
  we 
  come 
  to 
  the 
  question 
  of 
  waste 
  

   of 
  heat. 
  It 
  is 
  well 
  known 
  that 
  heat 
  is 
  lost 
  by 
  three 
  processes 
  : 
  conduc- 
  

   tion, 
  convection, 
  and 
  radiation. 
  Of 
  these, 
  metallic 
  conduction 
  would 
  

   be 
  very 
  important, 
  as 
  stated 
  before, 
  if 
  one 
  should 
  be 
  using 
  a 
  great 
  

   number 
  of 
  thermoelectric 
  elements, 
  but 
  it 
  is 
  possible 
  almost 
  entirely 
  

   to 
  eliminate 
  losses 
  by 
  conduction 
  with 
  boilers 
  of 
  certain 
  forms. 
  With 
  

   moderate 
  temperature 
  differences, 
  and 
  for 
  objects 
  in 
  the 
  open 
  air, 
  

   convection 
  is 
  a 
  far 
  greater 
  dissipater 
  of 
  heat 
  than 
  radiation. 
  But 
  

   direct 
  convection 
  may 
  be 
  eliminated 
  almost 
  altogether 
  if 
  the 
  body 
  

   to 
  be 
  cooled 
  is 
  enclosed 
  by 
  highly 
  evacuated 
  space 
  as 
  commonly 
  

   practiced 
  in 
  the 
  thermos 
  bottle. 
  In 
  that 
  case 
  cooling 
  proceeds 
  almost 
  

   wholly 
  by 
  radiation 
  from 
  the 
  inner 
  to 
  the 
  outer 
  wall 
  of 
  the 
  enclosing 
  

   evacuated 
  sheath, 
  and 
  thence 
  by 
  convection 
  and 
  radiation 
  to 
  the 
  

   surroundings. 
  This 
  consideration 
  leads 
  us 
  to 
  see 
  at 
  once 
  that 
  the 
  

   advantage 
  of 
  employing 
  an 
  evacuated 
  sheath 
  becomes 
  less 
  and 
  less 
  

   as 
  the 
  temperature 
  of 
  the 
  solar 
  boiler 
  rises 
  higher 
  and 
  higher. 
  For 
  

   radiation 
  increases 
  as 
  the 
  fourth 
  power 
  of 
  the 
  temperature 
  for 
  the 
  

   so-called 
  black 
  body 
  or 
  perfect 
  radiator. 
  Since 
  we 
  must 
  use 
  a 
  trans- 
  

   parent 
  sheath 
  to 
  admit 
  rays 
  to 
  the 
  boiler, 
  it 
  is 
  not 
  practicable 
  to 
  cut 
  

   down 
  radiation 
  by 
  fully 
  plating 
  the 
  inner 
  wall 
  of 
  the 
  evacuated 
  

   sheath, 
  as 
  in 
  the 
  thermos 
  bottle. 
  We 
  must, 
  therefore, 
  regard 
  the 
  inner 
  

   wall 
  of 
  the 
  sheath 
  as 
  approximately 
  a 
  "black 
  body." 
  Hence 
  the 
  inner 
  

   wall 
  of 
  the 
  evacuated 
  sheath, 
  when 
  at 
  high 
  temperatures, 
  will 
  radiate 
  

   strongly 
  to 
  the 
  outer 
  wall, 
  which 
  conducts 
  the 
  heat 
  to 
  its 
  outer 
  

   surface 
  and 
  there 
  loses 
  it 
  by 
  convection. 
  

  

  On 
  this 
  account 
  it 
  follows 
  that 
  although 
  the 
  sun's 
  temperature 
  is 
  

   so 
  high 
  that 
  boiler 
  temperatures 
  up 
  to 
  the 
  melting 
  point 
  of 
  materials 
  

  

  