Meter Bridge Video Lecture | Physics for EmSAT Achieve

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please make sure that you have watched
all the videos in part 18 before going
ahead with this one study about another
bridge which is similar to the
Wheatstone bridge known as the meter
bridge or sometimes it is also known as
the slidewire bridge so what does this
meter bridge basically meter bridge
looks somewhat like this you would have
seen this in your textbooks as well so
this is an apparatus which looks
somewhat like this and it is known as
metal bridge because it consists of a
wire whose length is one meter so it
basically comes first I will explain you
the construction it has a wire as you
can see here this is the wire this black
one this wire is of length one meter
okay and the ends of the wire are
stretched between two metallic strips so
this blue colored thing which you see
here this blue blue and blue they are
nothing but metallic strips so these two
metallic strips act as holder for this
wire this wire is kept straight and both
ends they are clamped to this metallic
strips so generally this metallic strip
is made up of some metal like copper
okay so there is another metallic strip
in between and it is arranged in such a
way that there is a gap in between so
that you can place two resistances here
so one of these resistance is unknown
resistance and the other resistance is
the unknown resistance so one resistance
you know the value of the resistance so
let us suppose this is the known
resistance and this s is unknown
resistance which you don't know the
value of these resistances now what do
you do there is a jockey here you know
what is a jockey it is basically an
object which which is capable of sliding
over this wire which is present there so
you have a
he there and one end off and the jockey
is connected to a galvanometer and the
other end of the galvanometer is
connected to this metal strip so that
means the galvanometer is connected
between this point and the point on this
wire under and the jockey which is
movable on the wire and there is a metre
scale which is held parallel to this
wire of length one meter so that we can
measure the length across it so what do
we do as we move the jockey on the this
wire as the jockey is moved what happens
the length of the wire let us suppose
for example let us say that the jockey
is at this point right now so if the
length of the wire this side is n 1 the
length along this side will be 100 minus
L 1 because this entire thing is 1 meter
that is 100 centimeters so if this
length is L 1 centimeters then this
length will be 100 minus L 1 centimeters
right let us suppose that our small R is
the resistance per unit length of the
wire so here whenever I talk of wine and
talking about this wire which is 1 meter
in length so let us suppose R is the
resistance per unit length of this wire
so that means if the length of this
portion of the wire is L 1 what is the
resistance of this portion this will be
L 1 into R right because R is nothing
but resistance per unit length so
resistance per unit length multiplied by
length will give you the resistance of
that part so you can calculate the
resistance of this part similarly you
can calculate the resistance of this
part now this circuit you can draw like
this your you have one resistance R you
have one resistance s you have one
resistant resistance r1 resistance s you
have one resistance here this part also
has some resistance this part also has
some resistance so let us call this
resistor
as X and this resistances Y so you have
one resistance X and one resistance Y
and you have a galvanometer which is
joined in between RS to in between X Y
that means there is a galvanometer which
is connected here from this point to
this point this is your galvanometer so
what does it look similar to doesn't so
you have it like this so this looks
similar to your wheat stone bridge right
you have four resistances and across
these two is connected the voltage
source so this becomes similar to the
wave stone bridge so using the principle
of the Wheatstone bridge what you can
write you can write that the bridge will
be balanced now let us suppose what you
do you keep moving this jockey and you
keep moving this jockey until that
deflection in the galvanometer becomes
zero
now as you keep moving you keep on doing
heat and try and suddenly at some point
you see that the galvanometer doesn't
show any deflection so then no
deflection in galvanometer that means
that R by s is equal to X by y then I by
s is equal to X by Y that is the bridge
is balanced bridge is balanced right now
what is this X and by X is nothing but
the resistance of this part so
resistance of this part will be
resistance per unit length into length
of this part divided by Y is resistance
of this part that is resistance per unit
length into length of this part right so
from this you can calculate the value of
the unknown resistance which comes out
to be 100 minus l1 divided by L
into app so this is how this method that
is the major bridge method or this MEC
meter bridge apparatus helps us in
evaluating the value of an unknown
resistance so I hope you understood the
arrangement you can see this arrangement
in north is it's lab in your schools as
well so what you basically do that
jockey which is fixed there you keep
moving the jockey as you will see that
the value of them current in the
galvanometer keeps changing and at some
point the galvanometer does not show any
deflection so that point indicates the
balanced condition of the bridge so at
that point the bridge is balanced so
using that you can apply the same
principle as that of the Wheatstone
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