er hand,
we can measure the potential difference in some known units, and
similarly measure the current flowing, we can determine the resistance
of a single electrolyte. The details of the apparatus may vary, but its
principle is illustrated in the following description. A narrow glass
tube is fixed horizontally into side openings in two glass vessels, and
an electric current passed through it by means of platinum electrodes
and a battery of considerable electromotive force. In this way a steady
fall of electric potential is set up along the length of the tube. To
measure the potential difference between the ends of the tube, tapping
electrodes are constructed, e.g. by placing zinc rods in vessels with
zinc sulphate solution and connecting these vessels (by means of thin
siphon tubes also filled with solution) with the vessels at the ends of
the long tube which contains the electrolyte to be examined. Whatever be
the contact potential difference between zinc and its solution, it is
the same at both ends, and thus the potential difference between the
zinc rods is equal to that between the liquid at the two ends of the
tube. This potential difference may be measured without passing any
appreciable current through the tapping electrodes, and thus the
resistance of the liquid deduced.

_Equivalent Conductivity of Solutions._--As is the case in the other
properties of solutions, the phenomena are much more simple when the
concentration is small than when it is great, and a study of dilute
solutions is therefore the best way of getting an insight into the
essential principles of the subject. The foundation of our knowledge was
laid by Kohlrausch when he had developed the method of measuring
electrolyte resistance described above. He expressed his results in
terms of "equivalent conductivity," that is, the conductivity (k) of the
solution divided by the number (m) of gram-equivalents of electrolyte
per litre. He finds that, as the concentration diminishes, the value of
k/m approaches a limit, and eventually becomes constant, that is to say,
at great dilution the conductivity is proportional to the concentration.
Kohlrausch first prepared very pure water by repeated distillation and
found that its resistance continually increased as the process of
purification proceeded. The conductivity of the water, and of the slight
impurities which must always remain, was subtracted from that of the
solution made with it, and the result, di