Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

From: Duff, A. Wilmer, A Text-Book of Physics, 5th ed., P. Blakiston's Son & Co., Philadelphia (1921) pp. 403-4.
© 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., Ex, (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. Ex 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., Ey, of a second galvanic cell. Hence
Ex : Ey :: 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.


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Last modified 22 July 2000