Page:Electrical Engineering Volume 1.djvu/270

1702 tendency represents the strong chemical affinity of zinc for oxygen and sulphur. Also, when acidulated water has been decomposed electrically, the separated oxygen and hydrogen tend to reunite.

2579. This tendency to reunite is strikingly shown by an electromotive force which is set up in the solution after the decomposing current ceases. This E. M. F. can be shown to exist by connecting a galvanometer in circuit with the decomposed solution. The deflection of this instrument will show that this E. M. F. due to chemical affinity acts in the opposite direction to the E. M. F. of the decomposing current. In other words, it is an opposing electromotive force.

2580. Careful measurement has shown that when hydrogen and oxygen combine with each other, an electromotive force of 1.47 volts is set up. From this it follows that no water can be decomposed unless an electromotive force of at least 1.47 volts is utilized; for it requires this much alone to offset the opposing E. M. F. of recombination.

2581. With every electrolyte there is a similar minimum E. M. F. necessary to produce continuous decomposition. This E. M. F. can be calculated for any electrolyte if the heat of formation and the electro-chemical equivalent of its constituents are known. The heat of formation is the thermochemical equivalent of the substance. By the thermochemical equivalent is meant the amount of heat liberated by the chemical combination of the molecular weight of one substance with another. This energy is usually expressed in gram-calories; that is, the amount of heat necessary to raise the temperature of one gram of water 1° Centigrade. This thermochemical equivalent is a value found by careful experiment. Thus, one gram of zinc, for instance, converted into zinc sulphate (ZnSO$4$), is known by experiment to give off about 4,000 heat-units as it combines. In Table 91 the heat of formation of various substances with oxygen is given.