
 From: Duff, A. Wilmer, A TextBook of Physics, 5th
ed., P. Blakiston's Son & Co., Philadelphia (1921) pp. 4034.

 © Copyright 1998 R. Paselk



 455. The Potentiometer.  This instrument in its simplest form consists of
a long uniform wire AB through which a constant current
flows from a battery M (Fig. 326). There is a fall of
potential from A to B and, the wire being uniform,
the fall of potential between two points is proportional to the
length of wire or the resistance between the two points. If two
points A and C on the wire be joined to a galvanometers,
there will be a current through AGC, as shown by the deflection
of the galvanometer. If we now introduce an opposing e.m.f.,
E_{x}, (a galvanic cell, a thermoelement, etc.)
in the galvanometer circuit, and find the point C, when
then is no current in the galvanometer we know that the fall
of potential between A and D is equal to the e.m.f.
E_{x} In the same way we find a point D,
such that the difference of potential between A and D
is equal to the e.m.f., E_{y}, of a second galvanic
cell. Hence

 E_{x} : E_{y} :: resistance AC :
resistance AD.
 :: length AC : length AD

 In this way two electromotive forces can
be compared and by using a standard cell, such as a Clark
or a Weston cell of known e.m.f., we can thus measure any
other e.m.f. In potentiometers of the highest precision, the
exposed wire is replaced by resistance coils in a box.


 © R. Paselk
 Last modified 22 July 2000