Incandescent Light

An electric current always heats the conductor through which it pa es to a degree which depends upon the strength of the current and the character of the conductor. The same current going through two wires, one of iron and the other of copper, of the same length and thickne , may heat the iron to a bright glow while the copper wire will be heated but a little. By properly regulating a current, it is po ible to bring any kind of a wire to incandescence, and keep it so; but to give much light a metal needs to be very near its melting point, and then it has so little elasticity and is so soft as to lose its shape. This difficulty hindered the early attempts to produce an electric light with currents from a galvanic battery. IMG:content-0756.png:[graphic] Fig . 2831. SHORT-CIRCUITING AN EIGHTY-LIGHT ARC MACHINE. negative, having what is called a crater excavated in it, while the other a umes a conical form, as shown in Fig. 28:30. Within this crater is to be seen a bright spot from which the arc starts. It is seldom as large as the tenth of an inch square, and from it comes sun were covered with such bright spots, it would radiate with as much energy as it now does. If more energy is spent upon an arc, it does not make it brighter or hotter; it makes the bright spot larger, and of course it radiates rmore, for its surface is greater.