Accurate representation of a model field in an electrolytic tank requires, among other things, that the electrical conductivity of the liquid should be everywhere constant. Einstein and Sander and Yates have discussed the importance of other possible inaccuracies, and have shown that by correct probe design, circuit design and choice of electrode materials, these may be made very small. This note describes very briefly some experiments, carried out in two shallow electrolytic tanks, which demonstrate that the principal error can be that introduced by variations of conductivity produced by non-uniformity of temperature distribution in the electrolyte. The point is of particular importance since the results also showed that the mechanical design of the tank can have a vital bearing on the achievement of uniform temperature.