The resistance of metals increases with the temperature. Consequently, a thin wire heated by the current will resist more and more and grow hotter and hotter until it loses heat by conduction and radiation into the surrounding air as rapidly as heat is supplied by the current. Thin wires heat much more rapidly than thick. The rise of temperature in different parts of a wire of uniform material but varying diameter (the current remaining the same) will be inversely proportional to the fourth power of the diameters. (a.) Suppose a wire at any point to become reduced to half its diameter. The cross-section will have an area 1/4 as great as in the thicker part. The resistance here will be 4 times as great, and the number of heat units developed will be 4 times as great as in an equal length of the thicker wire. But 4 times the amount of heat spent on 1/4 the amount of metal will warm it to a degree 16 times as great (16 = 2 to the 4th power). (b.) A thin platinum wire, heated white-hot by a current, is sometimes used in surgery, instead of a knife, as it sears the ends of the severed blood vessels and thus prevents hemorrhage. Platinum is chosen on account of its infusibility, but even platinum wires arc fused by too strong a current. Carbon is the only conductor that resists all attempts at fusion. (c.) Sometimes stout conducting wires are laid from a battery at a safe distance to a fuse connected with a blast of powder or other explosive. In the fuse, is a thin platinum wire, forming part of the electric circuit. The fuse is ignited by heating the platinum wire by sending the current through it. Such methods are frequently used in the operations of both peace and war.