ElectrodesTwo plates, often of platinum. The solution's resistance between them is measured.
SolutionThe electrolyte solution being tested. Its ions carry the current between the plates.
Distance between electrodesThe length l in the cell constant. More distance means more resistance.
Area of an electrodeThe area A in the cell constant. It is the area of one whole face. More area means less resistance.
The bridge 5
Conductivity cellIt is the unknown resistance of the bridge. The solution between its plates is being measured.
AC sourceIt feeds alternating current to two opposite corners of the bridge.
DetectorIt joins the other two corners. It reads zero when the bridge is balanced.
Variable resistanceIts resistance is known and can be changed. It is adjusted until the detector reads zero.
Fixed resistorsTwo resistors of known resistance. They form the two left-hand arms of the bridge.
How this list was compiled: Every name and fact checked on 2026-10-01, a name kept only where two independent sources use it. (1) Wikipedia, text only: Conductivity (electrolytic), https://en.wikipedia.org/wiki/Conductivity_(electrolytic) (the resistance of the solution is measured between two flat or cylindrical electrodes a fixed distance apart; an alternating voltage is generally used; resistance is proportional to the electrode spacing l and inversely proportional to the area A; the cell constant is the ratio of l to A and is found in practice with a solution of known resistance; solutions are traditionally measured in a Wheatstone bridge); Electrical conductivity meter, https://en.wikipedia.org/wiki/Electrical_conductivity_meter (electrodes often of platinum; alternating current reduces electrolysis and polarization of the electrodes); Wheatstone bridge, https://en.wikipedia.org/wiki/Wheatstone_bridge (four resistances as the sides of a diamond; two of known resistance, one adjustable, one unknown; the adjustable one is changed until no current flows through the meter that joins the two midpoints). (2) US National Institute of Standards and Technology, Special Publication 260-142 (2004 edition), Primary Standards and Standard Reference Materials for Electrolytic Conductivity, https://www.nist.gov/system/files/documents/srm/260-142-2ndVersion.pdf : resistance is proportional to the effective length between the electrodes, l, and inversely proportional to the effective cross-sectional area, A; the cell constant is l over A, and conductivity is the cell constant divided by the resistance; a cell is calibrated with standard solutions; the AC method uses a two-electrode cell with platinum disk electrodes and a Wheatstone bridge made for conductivity work, fed with alternating current by a signal generator, with equal ratio arms, decade resistors and a detector that shows the null point. (3) National Council of Educational Research and Training (India), Chemistry Part I for Class XII, Unit 2, Electrochemistry, section 2.4.1, https://ncert.nic.in/textbook/pdf/lech102.pdf : passing direct current changes the composition of the solution, so an alternating current source is used; the conductivity cell has two platinum electrodes coated with platinum black, of area of cross section A, separated by the distance l; l over A is the cell constant, found with a solution whose conductivity is already known; the bridge has two resistances, a variable resistance and the conductivity cell as the unknown resistance, is fed by an oscillator (a source of a.c. power) and is in balance when the detector carries no current. (4) A. W. Davison, H. S. Klooster and W. Bauer, Laboratory Manual of Physical Chemistry, experiment 37, read at https://archive.org/details/laboratorymunual031091mbp : the four arms of the bridge are two ratio resistors of fixed equal values, a variable resistance and the cell; the variable resistance is set so that no current flows through the telephone used as a detector; alternating current is used to avoid electrolysis. Detector is the word of sources 2, 3 and 4 and Variable resistance of 3 and 4; the two fixed arms are resistors of known resistance in 1, ratio-arms in 2, two resistances in 3 and ratio resistors of fixed equal values in 4. Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
A conductivity cell holds a solution between two electrodes so its resistance can be measured. The first view is also a cell constant diagram: the distance between the plates and the area of one. The second view is the bridge.
Read the full guide
The cell
Two plates, often of platinum, are fixed a set distance apart in the solution. The resistance between them rises with that distance and falls as the area of the plates grows. Distance divided by area is the cell constant.
The bridge
The cell is one arm of a Wheatstone bridge. Two arms are fixed resistors and the fourth is a variable resistance. An alternating current source feeds two corners and a detector joins the other two. The variable resistance is adjusted until the detector reads zero.
Conductivity is the cell constant divided by the measured resistance, so a known cell constant turns one reading into an answer.
Questions and answers
What is a conductivity cell?
A vessel that holds a solution between two electrodes fixed a set distance apart. The resistance of the solution between the electrodes is measured, and the conductivity is worked out from it.
What is the cell constant of a conductivity cell?
The distance between the electrodes divided by the area of an electrode, written l over A. In practice it is found by filling the cell with a standard solution whose conductivity is already known.
Why is alternating current used to measure conductivity?
A steady direct current would electrolyze the solution and polarize the electrodes, which changes the reading. An alternating current keeps both effects small.
How does the bridge measure the resistance of the solution?
The variable resistance is adjusted until no current flows through the detector. The bridge is then balanced, and the cell's resistance follows from the three known resistances.