Notes and News
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... ntent at http://about.jstor.org/participate--jstor/individuals/early-journal--content. JSTOR is a digital library of academic journals, books, and primary source objects. JSTOR helps people discover, use, and build upon a wide range of content through a powerful research and teaching platform, and preserves this content for future generations. JSTOR is part of ITHAKA, a not--for--profit organization that also includes Ithaka S+R and Portico. For more information about JSTOR, please contact firstname.lastname@example.org. 398 SCIENCE. SCIENCE. ductor of the current, the effect must be instantaneous; while, if the phenomenon resulted from some secondary action, it would probably go on increasing up to a certain point with the duration of the illumination, and it would also probably continue for a time after the light had been cut off. His method of making the test was equally simple and ingenious. The light was interrupted at rapid intervals by means of a rotating disk with holes or slits, and lbe placed a telephone in circuit with the battery. It is, then, obvious, that, if the effect is instantaneous, the telephone will pro(luce a note corresponding in pitch to the velocity of the disk; if other \vise, there will be silence. There was silence. A make and break in any other part of the circuit could be heard, but not in the beam of light: hence we must seek for some secondary action on the surface of the plates to explain M. Hallwach's experiments." THE ELECTRO-CHEMICAL EQUIVALENT OF SILVER. --A very imnportant electrical constant-one often used in the measurement of electric currentsis the amount of silver deposited in a given lime by a given electric current. Determinations have been madtle l)y Kohlrausch, Rayleigh, and Mascart, the results of the last differing from those of the first two by as much as one part in four hundred. Recently Pellat and Potier have repeated the exp!eriments, using to measure the currents an electro-dynamometer constructed by M. Pellat, and taking every precaution to insure accuracy. The result obtained gives 1.1192 milligrams of silver dleposited by one ampere in a second. The previous results are, Kohlrausch, I.1183; Rayleigh, I.I I8; Mascart, . I I 56; Pellat and Potier, I.I1192. The mean is very near to Rayleigh's value. THE VOLTAIC CURRENT OBTAINED WITH BISMUTH IN A MAGNETIC FIELD. -The following experiments are due to Dr. G. P. Grimaldi. A wi(le U-tube contained a solution of bismuth chloride in hydrochloric acid. In the two limbs of the tube dipped two wires of chemically pure bismuth very carefully polished. One limb of the tube was placed between the conical pole-pieces of a Faraday electro-magnet of medium size in such a way that the surface of the liquid was in the most intense part of the field. The two wires were joined up to a very sensitive Thomson astatic galvanometer. On closing the galvanometer circuit, a current was observecl which at first varied rapidly, but which finally reached a permanent value. This was compensated by means of a shunt containing a standard element, and the galvanometer was brought back to zero. If then the electro-magnet was excited by a powerful current, a permanent deflection of the galvanometer was observed; if the magnetizing current was broken, the galvanometer returned to zero. The current produced by magnetism, which the author calls the galvano-magnetic current, is independent of the intensity and direction of the current first observed in closing the galvanometer circuit before the magnet circuit is made. The latter is variable; the former is always in the same direction, -in the galvanometer circuit, from the magnetized bismuth wire to the nonmagnetized one; and in the liquid, from the non-magnetic metal to the magnetic one. The intensity of the galvano-magnetic current depends on the state of the surface of the metal, and to get regular results it is necessary to carefully polish the bismuth wires. To give an idea of the magnitude of the electro-motive force of the galvano-magnetic current, the author states that in the various experiments hitherto made under good conditions with various wires, and in various modifications, it has varied from -ToI to -fI0o of a Daniell cell, the magnetic field being produced by a Faradlay magnet of ordinary size, excited by a current of eight to twelve amperes, and with conical poles seven millimetres apart. With less powerful magnetizing currents, the results are smaller ; and, with a current of two amperes, the galvano-magnetic current is scarcely appreciable. The direction of the galvano-magnetic current is independent of the direction of the field : its intensity sometimes varied a little when the field was reversed, and sometimes remained constant.