Attention is directed to the many potential uses of specific electrical conductance (‘conductivity’) in the study of inland waters. Its measurement is capable of a precision useful in the detection of differences in a standing or flowing water-mass, but cannot be translated into measures of chemical concentration with equivalent absolute accuracy. Reasons — not infrequently neglected — include approximations in temperature correction, in allowance for a depression effect at higher ionic strength (salinity), and especially in the differences of specific conductance of chemically different ions. The last can be reduced by treating ionic concentration in chemical equivalents (e.g. meq L^-1) rather than the usual units of mass (e.g. mg L^-1) or molarity (e.g. mmol L^-1); also by making allowance for the exceptionally high equivalent conductance of H⁺and OH- ions of significance in markedly acid and alkaline waters.

Measurement in the field has been helped by the development of small portable instruments with inbuilt temperature compensation, flow-through electrode systems and electrical output. Examples of both field and laboratory measurements, for the charting and interpretation of various field situations, are illustrated chiefly from the author's experience. They include broad chemical surveys; interrelation with normal chemical analysis; longitudinal change, water travel and nutrient uptake in river and stream systems; ionic particulate conversion; horizontal and vertical differentiation in lakes; and ionic changes induced by photosynthesis.