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Moving contact C to the left or to the right position, when I is
2
not equal to zero, there will the current of certain direction in
galvanometer G appear. To determine the unknown EMF one strives

for current disappeared through galvanometer by moving the contact.
Then, instead of  normal element  with the known, EMF is
x N
connected and the scheme "is balanced” again (Fig. 3.2)

Figure 3.2

  I R (3.4)
N 1 A C

By doing equations (5) and (6) together, we’ll get the calculating
formula

R
 x   N A C (3.5)
R
AC

A slidewire is a metal string of a calibrated section with strong
resistivity, so resistance of a slidewire is proportional to its length i.e
R  l and R  l ,therefore
A C N AC x

l
   N N (3.6)
x
R
x

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