voltage   of   the   control   grid   varies   to   control   the strength  of  the  beam.  The  stronger  the  beam,  the brighter  the  display  is  on  the  phosphor  screen. The  screen  grid  voltage  remains  constant  and  acts as  an  accelerator  for  the  beam.  A  negative  charge  on the   focus   grid   shapes   the   electrons   into   a   beam. Varying  the  charge  of  the  focus  grid  causes  the diameter  of  the  beam  to  vary  to  determine  optimum focus. Deflection   Systems The  deflection  system  in  a  CRT  moves  the  beams to   create   the   display. Two   common   types   of deflection   systems   are   used   in   CRTs.   These   are electromagnetic deflection and  electrostatic deflection. ELECTROMAGNETIC   DEFLECTION.–   Elec- tromagnetic  deflection  uses  a  magnetic  field  generated by  four  coils  to  move  the  beam  across  the  CRT. Electromagnetic   deflection   is   commonly   found   on CRTs  that  use  a  raster-scan  type  display. Current  flows  through  the  electron  beam  as  it moves   from   the   electron   gun   (cathode)   to   the phosphor  face  (anode)  of  the  CRT.  This  current develops  a  circular  magnetic  field.  By  introducing  an external   magnetic   field,   the   beam   can   be   deflected. Controlling  the  polarity  and  strength  of  this  external field  controls  the  amount  and  direction  of  the  beam deflection. The  magnetic  field  is  introduced  into  the  CRT  by the  yoke  assembly.  The  yoke  consists  of  four  coils  of wire   mounted   at   90-degree   increments.   The   yoke   is mounted   around   the   neck   of   the   CRT.   Current flowing  through  the  coil  produces  a  magnetic  field  at a  right  angle  to  the  coil.  The  magnetic  field  will  cause the  electron  beam  to  deflect. ELECTROSTATIC   DEFLECTION   CRT’S.– Electrostatic-type  deflection  CRTs  are  generally  used in   radar   and   oscilloscopes. In  the  electrostatic deflection   CRT,   four   deflection   plates   are   located inside   the   CRT.   The   top   and   bottom   plates   control vertical  deflection  of  the  beam  and  the  right  and  left plates  control  the  horizontal  deflection  of  the  beam. An  electrical  charge  is  applied  to  these  plates  to  direct the  beam  to  the  proper  area  of  the  CRT.  To  move  the beam  to  the  right,  a  positive  charge  is  applied  to  the right  plate  to  pull  the  beam  while  a  negative  charge  is applied  to  the  left  plate  to  push  the  electron  beam  to the   proper   position. The   amount   of   the   charge applied  to  the  plates  controls  the  amount  of  deflection. CRT  SCANNING  METHODS The  creation  of  a  display  is  known  as  a  scan.  Two types  of  scanning  systems  are  currently  in  use  in CRTs:  raster  scanning  and  vector  scanning.  Raster scan  CRTs  are  commonly  used  with  electromagnetic deflection  CRTs.  Vector  scan  CRTs  are  commonly used   with   electrostatic   deflection   systems,   although either  deflection  system  can  be  used  with  either scanning   system. Raster   Scanning A  raster  scan  CRT  develops  the  display  or  picture by  painting  a  series  of  horizontal  lines  across  the  face of  the  CRT.  The  electron  beam  is  pulled  from  left  to right.  The  beam  is  then  turned  off  and  the  horizontal deflection  voltage  returns  the  beam  to  the  left  side, and  the  vertical  deflection  voltage  pulls  the  beam down  one  line  space. The left to right motion is the horizontal frequency and  is  much  greater  than  the  top  to  bottom  motion  or vertical  frequency.  The  time  it  takes  for  the  beam  to return  to  the  left  or  top  of  the  screen  is  known  as retrace  time.  During  retrace  the  beam  is  blanked. By   dividing   the   horizontal   frequency   by   the vertical   frequency,   we   can   determine   the   maximum number  of  lines  in  the  raster.  Standard  television  uses 15,750  Hz  for  the  horizontal  frequency  and  60  Hz  for the   vertical   frequency.   Using   this   formula,   we   find that  the  maximum  number  of  lines  is  262.5;  but  some lines  are  not  available  because  of  the  time  required  for vertical   retrace. The lines are spaced close enough to each other so the  eye  cannot  detect  any  variation  of  intensity. Resolution   is  the  number  of  lines  per  inch  at  the 1-3