not directly in contact, but are placed in a vessel filled with water. As we are only dealing with potential differences, we may arbitrarily fix the potential of one terminal at zero. Let this be the end of the copper strip soldered at a to the zinc plate. Then the potential of the zinc plate, which is due to the contact e.m.f. of the junction a where the copper strip is soldered to it, will by the table on p. 47 be 0.738 volts. Since by hypothesis the water is inert both as regards the zinc and the copper, this will also be the potential of the copper. The junction at b cannot alter this value, since at that place two equal metals are in contact. The potential at B is therefore also 0.738 volts, and on joining A with B by a wire a current will flow. Now let us replace the water by a dilute solution of sulphuric acid. The difference of contact e.m.f. of this liquid in relation to zinc on the one hand and copper on the other is about one-third of a volt, and this difference acts in the same sense as the contact e.m.f. at a. The result is that the potential of the copper plate, and therefore also of its terminal B, has now been increased to a little over one volt. Here we have a combination of substances, which, by virtue
