In this paper the authors discuss their research on heat generation of electric contacts named "Hot-zone phenomenon". When a large electric current flows in an electric wire jointing section of abnormal conditions, the temperature of the section rises and subsequently causes an electric fire. We have previously observed this Hot-zone phenomenon and explained the growth mechanism of the condition in an A. C. circuit. In this paper we have conducted experiments on the conditions of a D. C. circuit of about 12 working voltage using copper electric wires. The experiments were carried out by observing abnormal conditions caused by the opening and closing action at the electric wire jointing section. The growth mechanisms of the Hot-zone phenomena were then explained more precisely. The results obtained agreed approximately with those of an A. C. circuit as expected.
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