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118 grown reckless in their calling, often resort to this simple test to find out whether a wire is what is technically termed "alive," but no amount of shocks will ever enable a man to say how many amperes are flowing in the wire. This can only be determined by observation of certain effects produced by the current. These effects may be chemical, thermal, magnetic or mechanical.

Chemical action of an electric current.—The substance of a copper wire carrying a current undergoes no change. It may get heated whilst the current flows, but chemically it remains unaltered. Even if the wire is an alloy of two metals there is no change in its chemical nature. Also with liquid conductors, such as mercury or molten iron, the passage of a current does not produce a chemical change, but if the liquid is some chemical compound there is such a change. Imagine two copper plates placed in a solution of copper sulphate and provided with terminals AB, such as shown in Fig. 1, p. 48. Attach to these terminals the wires leading to a dynamo machine or some other source from which an electric current flowing always in the same direction may be obtained. Let us also put into the circuit some instrument