TM 1-1500-204-23-4 Figure 2-5.  Series Circuit Figure 2-7.  Compound Circuit d. Ammeter.  An ammeter measures either dc or ac current.    To  measure  current,  the  circuit  under  test  is broken,  and  an  ammeter  is  inserted  into  the  break.    Its scale    is    usually    graduated    in    kiloamperes,    amperes, milliamperes, or microamperes. e. Voltmeter.    A  voltmeter  is  a  current  measuring instrument designed to indicate voltage by measuring the current  flow  through  a  resistance  of  known  value.    Its scale is usually graduated in volts.  D'Arsonval voltmeters are being phased out by digital voltmeters. f. Ohmmeter.      An   ohmmeter   measures   electrical resistance   and   is   used   to   check   circuit   continuity.      Its scale is usually graduated in ohms, megaohms, or both. g. Multimeter.        A    multimeter    is    an    instrument incorporating   two   or   more   meter   circuits   and   a   meter movement in a single case.  A typical multimeter contains voltmeter, ammeter, and ohmmeter circuits using a single meter  movement.    The  function  switch,.    as  shown  in figure 2-10, must be placed in the proper position for the type of measurement to be made.  The range switch must be set to the range desired. h. Electrodynamometer.      The   electrodynamometer or  wattmeter  generally  is  used  to  measure  power  taken from   the   line   for   operation   of   any   piece   of   electrical equipment. i. Rectifier Meter.  Low-frequency ac measurements are  made  with  a  rectifier  meter.    The  rectifier  meter  is more sensitive and more accurate than the moving vane D'Arsonval-type meters.  The function of a rectifier meter is to change ac to a form of dc.  The ac is rectified, and the pulsating dc is measured by the meter. j. Wattmeter.  The wattmeter (electrodynamometer) is used to measure power in an electrical circuit.  Its scale is  usually  graduated  in  watts.    The  wattmeter  consists  of two stationary coils and one moving coil.  The stationary coils have many turns of small wire with high resistance. The  movable  coil  has  a  few  turns  of  large  wire  with  low resistance and is pivoted in jewelled bearings.  The meter needle  is  attached  to  one  end  of  the  movable-coil  shaft, and   the   damping   arrangement,   similar   to   that   of   the moving-vane meter, is attached to the other end.  Springs are provided to control the needle swing. k. Frequency   Meters.      Frequency   or   wavelength, however, may be determined also by frequency measuring     devices     known     as     wavemeters     or     beat frequency meters. These meters indicate the fundamental   or   harmonic   frequencies   of   oscillators   or harmonic generators on a calibrated dial.  The wavemeter is   calibrated   in   terms   of   wavelength,   and   contains   a variable    tuned    circuit    whose    resonant    frequency    is determined   by   the   unknown   frequency.      The   beat,   or heterodyne    frequency,    meter    uses    an    oscillator    to generate   signals   of   known   frequencies   and-compares these with the unknown frequency. 2-7