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Into the same circuit he introduced his voltameter, which consisted of a graduated tube filled with acidulated water and provided with platinum plates for the decomposition of the water, and also a cell containing chloride of tin.
The fact that the weight of metal deposited in this way from its salts is proportional to the current, has been utilised for measuring the strength of currents with a fine degree of accuracy. If, for example, the tubes of the voltameter described on page 38 were graduated, the volume of gas evolved would be a measure of the current.
With the indications of his voltameter he compared the decompositions of other substances, both singly and in series. He submitted his conclusions to numberless tests. He purposely introduced secondary actions. He endeavoured to hamper the fulfilment of those laws which it was the intense desire of his mind to see established.
In order to be able to measure the amount of this action, he invented an instrument which he called a voltameter, or a volta-electrometer. It consisted of a simple device for measuring the amount of hydrogen and oxygen gases liberated by the passage of an electric current through water acidulated with sulphuric acid.
We have the fact that light falling on the platinum electrode of a voltameter generates a current, first observed, I think, by Sir W. R. Grove at any rate, it is mentioned in his "Correlation of Forces" extended by Becquerel and Robert Sabine to other substances, and now being extended to fluorescent and other bodies by Prof. Minchin.
Having seen how electricity can be generated and stored in considerable quantity, let us now turn to its practical uses. Of these by far the most important are based on its property of developing light and heat as in the electric spark, chemical action as m the voltameter, and magnetism as in the electromagnet.
Completing his circuit, he permitted the current to continue until 'a reasonable quantity of gas' was collected in the voltameter. The circuit was then broken, and the quantity of tin liberated compared with the quantity of gas. The weight of the former was 3.2 grains, that of the latter 0.49742 of a grain. Oxygen, as you know, unites with hydrogen in the proportion of 8 to 1, to form water.
Experiments already referred to had taught him that this substance, though an insulator when solid, is a conductor when fused, the passage of the current being always accompanied by the decomposition of the chloride. He wished to ascertain what relation this decomposition bore to that of the water in his voltameter.
He showed, by numerous experiments, that the decomposition effected is invariably proportional to the amount of electricity passing; that variations in the size of the electrodes, in the pressure, or in the degree of dilution of the electrolyte, had nothing to do with the result, and that therefore a voltameter could be employed to determine the amount of electricity passing in a given circuit.
Their experiment is usually performed by the. apparatus shown in figure 20, which is termed a voltameter, and consists of a glass vessel V, containing water acidulated with a little sulphuric acid to render it a better conductor, and two glass test-tubes OH inverted over two platinum strips or electrodes, which rise up from the bottom of the vessel and are connected underneath it to wires from the positive and negative poles of the battery C Z. It will be understood that the current enters the water by the positive electrode, and leaves it by the negative electrode.
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