of   the   recorder.   Set   the   record   MIKE/PA- MONITOR   slide   switch   to   RECORD   MIKE. Select  the  desired  tape  speed,  usually  7-1  /2  or 3-3/4 ips (19 or 9.5 ems), by means of the speed selector. If the recorder you are using is designed to use 4-track recordings on the tape, select the track on which you wish to record. This is done by setting the track selector to the proper track position. If it is desired to set the recorder level before the tape is set in motion, turn the tone control to TREBLE  and  adjust  the  volume  control  until the  “Normal”  half  of  the  indicator  flashes  and no  flashing  occurs  at  the  “Distorted”  half. Pull  forward  the  instant/stop  record  lock lever  and  press  the  record  key.  Release  the  in- stant  stop  lever,  and  recording  will  start.  When the recording is finished, press the stop key. The  signal  can  be  monitored  during  record- ing if the slide switch on the rear panel is set to PA-MONITOR.  However,  be  careful  to  keep the   microphone   away   from   the   speaker,   or acoustic   feedback   may   cause   squealing.   This feedback  can  be  avoided  if  earphones  are  used instead  of  the  speaker.  Plug  the  earphones  into the external speaker socket. When the recording is  completed,  rewind  the  tape  by  moving  the high-speed lever to the left. Before rewinding the tape,   you   may   record   additional   material   by turning  over  the  full  takeup  reel  and  placing  it on the supply spindle. RECORDING   FROM   AN   EXTERNAL AMPLIFIER.—Recording   can   be   made   from external  sources  such  as  phonographs,  radios, television  sets,  AM-FM  Tuners,  or  other  tape recorders.  The  outputs  from  these  sources are   commonly   marked   “Detector,”   “Tape,” “Recorder    Input,”  or  “Preamp  Output,”  and can   be   connected   directly   to   the   microphone- phonoradio  input  of  the  recorder.  These  exter- nal sources are connected to the input jack with a long (1-3/16 inch, 30-mm) plug. After the con- nection, the procedure is the same as when you record with a microphone. If the external ampli- fier has a level control for the output jack being used, it should be set so that the recorder volume will  be  somewhat  near  its  midrange  for  proper operation  of  the  level  indicator.  Tone  controls and  loudness  controls  on  the  external  amplifier should be set as nearly as possible to feed a flat (or  high-fidelity)  output  signal  to  the  recorder. ERASING  THE  TAPE.—When  a  monaural recording   is   made,   any   previously   recorded material   on   the   tape   is   automatically   erased before  the  new  material  is  recorded.  Erasing  is done only when the recorder is in the RECORD function.  A  half-track  head  (in  all  models  that use half-track heads) erases only that half of the tape  that  is  being  used;  a  quarter-track  head (characteristic  of  many  models  with  4-track recording  capability)  erases  only  one-fourth  of the  tape  when  a  recording  is  being  made.  To erase a track without recording new sound, turn the   volume   control   down   before   pressing   the record  key. In  all  models  designed  to  use  4-track  tapes, the tape should be erased before recording if the tape  will  be  played  on  a  recorder  which  has  a half-track head. This is necessary because during a  recording  such  a  model  simultaneously  erases and  records  only  one-fourth  the  width  of  the tape (figure 7-21C); so there will be both old and new  recordings  on  half  of  the  tape  the  second recorder  would  play.  Erasing  can  be  done  by running the tape through the recorder four times or by the use of a bulk tape eraser. The purpose of  the  erase  head  is  to  remove  any  previously recorded  signal  before  the  tape  reaches  the record  head.  The  erase  head  acts  as  an  elec- tromagnet,  with  a  field  powerful  enough  to destroy  whatever  magnetic  patterns  happen  to exist  on  the  tape.  As  a  given  point  on  the tape  recedes  from  the  erase  head,  the  powerful field   in   effect   diminishes,   gradually   bringing that point on the tape to a demagnetized condi- tion. A  tape  consists  of  a  coating  of  iron  oxide  on a  base  of  mylar  plastic.  The  iron  oxide  is  easily magnetized  and  demagnetized.  When  the  oxide is  subjected  to  the  magnetic  field  of  the  record head,   the   result   is   to   create   a   series   of   bar magnets  of  varying  lengths  and  depths  on  the tape.  Long  magnets  correspond  to  low  notes; short  ones  to  high  notes.  Deep  magnets  corre- spond  to  strong  recorded  signals;  shallow  ones to  weak  signals.  Each  bar  magnet  has  a  south seeking  pole  and  a  north  seeking  pole,  with  an external magnetic field between the two poles. In playback the fields of the bar magnets excite the playback  head. The   prevalent   distinction   among   types   of tape  is  in  terms  of  thickness  which  determines the  amount  of  tape  that  can  be  wound  on  a 7-40