16-mm  film.  The  16-mm  films  are  used  exten- sively  for  training,  briefings,  and  entertaining personnel. The  16-mm  motion  picture  projectors  can  be operated   virtually   anywhere   that   105-125V (50-60Hz)  alternating  current  is  available.  Most of  these  projectors  will  project  silent  or  sound motion   pictures.   There   are   some   projectors however,  that  project  either  silent  or  sound  but not  both. Most  projectors  are  equipped  with  built-in sound  systems  capable  of  working  with  or without   separate   amplification   equipment. Usually, a speaker is supplied with the projector. In  many  models  the  amplifier-loudspeaker  com- ponents  are  mounted  in  the  projector  case. Principles of Operation The  operation  of  the  various  makes  and models   of   motion   picture   projectors   is   very similar.  Read  the  instruction  manual  for  each different   projector   before   operating   that   par- ticular equipment. SETTING  UP  THE  PROJECTOR.—When using a portable screen, set it up relative to the audience  size.  Position  the  projector  stand  or sturdy table at the approximate correct distance from  the  screen.  Place  the  projector  on  the stand.  It  must  be  high  enough  so  that  the  pro- jected  light  will  be  above  the  heads  of  the audience to prevent shadows on the screen. Con- nect  the  power  cord  to  a  three-prong  outlet.  If the  presentation  is  with  sound,  turn  on  the volume  control  to  warm  up  the  amplifier  tubes. (NOTE:   All   transistor   amplifiers   require   very little warmup.) Most reel arms are permanently mounted to the  projector  requiring  only  that  you  raise  and lock  them  into  place.  In  older  models  however, the  reel  arms  must  be  assembled;  if  so,  remove them from the carrying case and secure them to the  projector. Place  the  reel  holding  the  film  on  the  front arm and an empty reel on the rear arm. Figure  7-15  is  a  side  view  of  a  manual threading  projector  and  is  typical  of  many  pro- jectors   in   use.   Study   this   figure   to   become familiar  with  the  important  parts  of  the  projec- tor. MANUAL   THREADING.—Most   projec- tors   have   a   threading   diagram   displayed somewhere  on  the  projector  body.  Find  detailed threading  instructions  in  the  manufacturer’s instruction  manual  for  the  particular  make  and model  projector  you  are  using.  Be  sure  that  the sprocket  holes  are  engaged  with  the  sprocket teeth.  It  is  also  important  that  you  form  proper sized  loops  above  and  below  the  film  gate.  The upper   loop   permits   intermittent   movement   of the  film.  The  lower  loop  prevents  this  irregular motion  of  the  upper  loop  from  affecting  the smooth  feeding  of  film  through  the  sound  head. AUTOMATIC    THREADING.—Automatic threading greatly simplifies your task of prepar- ing  for  a  presentation.  The  following  steps  are used  to  thread  a  projector  with  the  automatic threading  feature:  First,  ensure  that  the  end  of the  film  leader  is  squarely  cut.  Move  the threading  control  lever  to  the  load  position. Insert the end of the leader into the input chan- nel, engaging it with the upper sprocket. Switch the  projector  to  run.  The  projector  will  thread automatically. When the leader exists at the rear of  the  projector,  turn  the  projector  off.  Tug gently   on   the   end   of   the   leader.   This   action releases   the   automatic   threading   mechanism. Wrap the film to the takeup reel. This completes the  threading  operation. STILL   PICTURES.—Some   projectors   are designed  to  stop  on  individual  frames,  per- mitting  them  to  be  shown  as  still  pictures.  To show a single frame, rotate the still picture con- trol  to  STILL  or  depress  the  STILL  button, whichever  is  appropriate  for  the  specific  projec- tor.  Frequently  you  must  refocus  to  bring  out  a sharp   still   picture. A   heat   shield   will automatically  drop  into  position  protecting  the film  from  heat  damage.  This  shield  blocks  off much of the light and dims the picture. (Full in- tensity  of  light  from  the  projector  lamp  would heat  the  film  to  its  melting  point  in  just  a  few seconds.)  Turning  the  control  switch  to  STILL or  depressing  the  STILL  button  removes  power from  the  the  drive  motor,  stopping  the  main drive  shaft.  The  heat  shield  is  then  pulled  into the optical path by the return spring. 7-26