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CURRENT ELECTRICITY


                   Galvanometer is very sensitive instrument and can detect
                   small  current  in  a  circuit  (Fig.14.3).  A  current  of  few
                   milliamperes is sufficient to cause full scale deflection in it.
                   While making the connections polarity of the terminals of
                   the  galvanometer  should  be  taken  into  consideration.
                   Generally,  the  terminal  of  the  galvanometer  with  red
                   colour shows the positive polarity while that of with black
                   colour shows the negative polarity. An ideal galvanometer
                   should have very small resistance to pass the maximum
                   current in the circuit.
                   After suitable modification galvanometer can be converted       Fig.14.3: A galvanometer
                   into an ammeter (Fig. 14.4).  A large current of the range such
                   as 1 A or 10 A can be measured by means of ammeter. Like
                   galvanometer, ammeter is also connected in series, so the
                   current  flowing  in  the  circuit  also  passes  through  the
                   ammeter (Fig.14.5).
                        Battery                    Knife switch

                                                                  Ammeter







                                                                                    Fig.14.4: An ammeter
                                           Light bulb
                                                           Electric current
                     Fig.14.5: Schematic diagram showing the measurement of current

                   14.2      POTENTIAL DIFFERENCE
                   When one end A of a conductor is connected to the positive
                   terminal and its other end B is connected to the negative
                                                                                       Do you know?
                   terminal of the battery, then the potential at A becomes      The  galvanometer  has  been
                   higher than the potential at B (Fig.14.6).                    named  after  Luigi  Galvano
                                          Direction of current                   (1737-1798).  He,  while
                                                                                 dissecting  a  frog's  leg,
                                                V
                                                                                 discovered  that  dissimilar
                                   A                         B                   metals touching the leg caused
                                                                                 it  to  twitch.  This  chance
                                                                                 discovery, the invention of the
                                                                                 chemical cell and the battery.
                                         Direction of electrons
                                      K       +  -
                                                V
                                            Fig.14.6
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