Another principle is adopted in the electrical instruments which expose to heat a junction of two different materials, usually metallic, giving rise to an electric current, easily measured. Experience shows that the most satisfactory couples for temperatures between 300° C. (570° F.) and 1600° C. (2900° F.) are those devised by M. Le Chatelier, one half consisting of pure platinum, the other half an alloy of ten per cent. rhodium and ninety per cent. platinum. Such instruments are indispensable in the arts which employ high temperatures. In producing chlorine by the Deacon process, or in the baking of porcelain, an undue variation of temperature of only twenty degrees may cause a complete failure of the operation. The Measurement of Light. It is probable that about one half the electricity from the dynamos of America is sent into lamps, and this is but part of the whole outlay for light, still chiefly produced by petroleum and gas. Hence the importance of measuring the light from lamps, jets, and mantles of various kinds, and testing the efficiency of shades and reflectors. First of all comes the decision as to a standard for comparison. Great Britain has adopted the Harcourt lamp, consuming pentane, as a standard for ten candle-power, referring to the old time candle of spermaceti. Germany employs the amylacetate lamp introduced by Von Hefner Alteneck, as a standard for its Hefner unit of illumination. Both lamps share in a difficulty which attends all combustion: atmospheric conditions which vary from hour to hour, from place to place, greatly affect the intensity of a flame. Hence incandescent lamps, which have been compared with these fundamental standards, are used as working standards. They can be operated by a uniform current of specified voltage, and after a hundred hours’ use their constancy of radiation for a considerable period is remarkable. [Illustration: Photometer. A, standard candle. B, gas flame. S, sliding frame.]