06 J. A. EWING AND J. G. MACGREGOR ON THE 
After him came WIEDEMANN (Pogg. Ann., xcix., 1856). He examined one 
or two salts carefully. He gives no formula. 
Next comes Brecker (Ann. der Chem. und Pharm., 1850 and 1851). His 
experiments were chiefly on the effect of increased temperature. He gives an 
elaborate formula, in which the resistance appears as an expansion of the first, 
second, and third powers of the temperature, and also of the amount of salt in 
solution. 
The most extensive series of experiments on sulphate of zine were those of 
Beetz (Pogg. Ann., cxvii, 1862). His only precaution against polarisation 
was the use of zinc electrodes, which, curiously enough, he amalgamated. His 
investigations of the relation between conductivity and temperature are very 
valuable. Unfortunately, in the other part of his work—the connection between 
conductivity and density—he was not careful to keep to exactly the same 
temperature throughout a whole series of solutions, so that his results do not 
admit of accurate graphic representation. He gives the conductivity in the 
form of an expansion of the first, second, and third powers of the amount of 
salt in solution, and does not appear to have arrived at any more simple 
relation between them. 
The electromotive force caused by the polarisation of the electrodes reaches 
a maximum in every case, and, however great a decomposing electromotive 
force be used, this can never be exceeded. One form of apparatus, based on 
this fact, was a trough with parallel plates of platinum for electrodes, the distance 
between which could be varied at pleasure. The current was first measured 
by a tangent galvanometer, when the trough alone was in circuit along with 
a sufficient number of cells to produce the maximum polarisation. The distance 
between the plates was then reduced, a metallic resistance being introduced into 
the circuit and adjusted till the deflection of the galvanometer needle was the 
same as before. Then the resistance of the coils which had been introduced 
was equal to that of a column of liquid, whose cross section was the area of the 
plate, and whose length was the difference of the distances between the plates 
in the first and second experiment. The effect of polarisation, being the same 
in both cases, was eliminated. The electromotive force, however, which was 
required to produce the maximum polarisation, was so high as to cause rapid 
electrolysis, which not only changed the constitution of the liquid during the 
time occupied by the test, but also introduced an element of error due to the 
continually varying resistance of the bubbles of gas which formed on the plates. 
In order to remedy these defects, KonLRAUSCH and NippoLpT (Gottingen Nachr., 
1868 ; Jahresbericht des phys. Vereins zu Frankf., 1867-68 ; also Pogg. Ann., 
cxxxviii., 1869) employed induced currents from a magneto-electric machine, 
which followed each other in rapid succession in opposite directions. The 
electromotive force of these currents was reduced by means of a thermoelectric 
