7. WURTS’S EXPERIMENT. NON-ARCING METALS AT HIGH VOLTAGE. _Trans. Amer. Inst. Elect. Eng._ March 15, 1892. _Ann. Chem. Phar. Sup._ VII, 354 and VIII, 133. _Chem. News_, VII, 70; X, 59, and XXXII, 21, 129.—Mendelejeff and Meyer discovered that chemical elements occur in natural groups by a principle which they termed the periodic law. One of these groups includes zinc, cadmium, mercury and magnesium; and another group, antimony, bismuth, phosphorus and arsenic. Alex. J. Wurts, of the Westinghouse Electric Co. found that the metals of these groups are non-arcing, by which he means that with an alternating current dynamo of a thousand or more volts, and with the said metals as electrodes in the air only just escaping each other, it is impossible to maintain an arc as in the case of an ordinary arc lamp having carbon electrodes or in a lightning arrester usually having copper electrodes. He suggested and theorized that certain chemical reactions served to explain the phenomena. With low voltage—as 500, the arc was maintained between all metals. § 6. A two pole lightning arrester is shown in Fig. III The arc formed, ceased instantly. One of the best metals for practical use is an alloy of 1/2 zinc and 1/2 antimony, or any metal electroplated with a non-arcing metal. Freedman observed a critical point with electrodes of brass. The current was gradually reduced until the arc became like the discharge of a Holtz machine whose condensers have been disconnected. See _Elect. Power_, N.Y., Feb. 1896, p. 119. 8. WHEATSTONE’S EXPERIMENT. DURATION OF SPARK. _Phil. Tran._ 1834.—The short duration of an electric spark produced by a single disruptive discharge is easily made apparent by a rapidly rotating disc, having radial sectional areas of different colors. With reflected sunlight, the colors seem to blend into one tint upon the principle of the persistence of vision; (See Swain’s experiment. _Trans. R. So. Edin._ ’49 and ’61.); but when viewed by the flash of a spark, the colors are seen as distinctly separated as if the disc were at rest. By calculation, based directly upon a series of experiments, he found the duration of the spark to be about .000042 sec. It was discovered also, by the rotating mirror, that the apparently single spark was composed of several following each other in quick succession, and he concluded that the current during the discharge was intermittent. He considered each of the divisions of the spark as an electric discharge. Prof. Nichols, of Cornell University, and McKittrick obtained curves indicating the variation of E. M. F. during the existence of a spark. _Trans. Amer. Inst. Elect. Eng._ May 20, ’96.