TM 1-1500-204-23-4 Figure 2-1.  Direct Current Wave Form Figure 2-2.  Alternating Current Cycle the positive terminal of the battery.  It provides only one possible  path  for  current  to  flow.    Current  flow  passes through   circuit   components,   battery,   and   resistor,   one after the other, or in series. b. Parallel.  A circuit in which two or more electrical resistances,   or   loads,   are   connected   across   the   same voltage source, as shown in figure 2-6, is a parallel circuit. The  parallel  circuit  differs  from  the  series  circuit  in  that more  than  one  path  is  provided  for  current  flow.    The minimum    requirements    for    a    parallel    circuit    are    the following: · A power source. · Conductors. · A resistance or load for each current path. · Two or more paths for current flow. c. Compound.  A compound circuit is a combination of  series  and  parallel  circuits.    A  series-parallel  circuit consists of groups of parallel resistors.  An example of a series-parallel    circuit    is    shown    in    figure    2-7.        The requirements for a series-parallel circuit are as follows: · Power source (battery). · Conductors (wires). · Load (resistances). · More than one path for current flow. · A control (switch). · Safety device (fuse). 2-6.  Measuring Equipment.  Various lights, testers, and meters used to measure electrical values are explained in the following paragraphs. a. Test  Lights.    Test  lights  consist  of  ordinary  low voltage incandescent lamps, neon lamps or headsets, and a pair of leads for connecting the indicator to the circuit to be   tested.      These   testers   are   simple   pieces   of   test equipment used to.  check the continuity of fuses and line circuits.    These  testers  do  not  give  accurate  qualitative measurements  such  as  can  be  obtained  with  a  meter. However,   their   simplicity   is   of   considerable   advantage when open and closed circuit tests are made.  The neon and lamp testers also can be used to distinguish between ac and dc supplies and to test capacitors.  In some testers a switching arrangement allows a source of voltage to be inserted  in  series  with  the  test  lamp,  so  that  circuits  with no voltage applied to them can be checked. b. Continuity  Tester.    A  continuity  tester  uses  the constant-current circuitry of a multimeter for measurements of resistance.  It can be used to check for opens, shorts, or grounds, as shown in figure 2-8. c. D'Arsonval   Meter .      The   D'Arsonval   meter,   as shown in figure 2-9, is a dc galvanometer consisting of a narrow rectangular coil suspended between the poles of a permanent  magnet.    The  D'Arsonval  meter  movement  is a  current  measuring  device  which  is  used  in  ammeters, voltmeters,   and   ohmmeters.      The   D'Arsonval   meter   is being phased out by digital measuring equipment. 2-3