LAB # 34: WHEATSTONE'S BRIDGE
INTRODUCTION:
The Wheatstone bridge provides an accurate way of comparing an unknown resistance to
a known resistance and thus measuring the unknown. See Fig.1.
When the bridge has been balanced, i.e., when the current through the galvanometer is
zero, the four resistance's in the bridge are related by the simple equation:
R
x
R
s
=
L
2
L
1
In this equation, L
1
and L
2
are the lengths of the slide wire to left and right of the sliding
contact. Normally the resistance R
2
and R
1
would enter, but here they are simply
proportional to L
2
and L
1
, which we can easily measure. R
x
is the unknown resistance and
R
s
is the standard.
The best precision can be obtained if the standard and unknown resistance's are
approximately equal. Then the sliding contact will be near the center of the slide wire,
and L
1
and L
2
will be approximately equal, i.e., a small error in reading L
1
and L
2
will not
produce a large change in their ratio. See Fig 2.
Thus, as you proceed to balance and see that a particular choice of R
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 Fall '08
 RABE
 Physics, Current, Resistance, Heat, Measuring instrument, slide wire

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