482 SCItNTIFlC RECORD FOR 1884. 



chanically a current is developed. {C. B., May, 1884, xcviii, 1256; J. 

 Phys., September, 1884, II, iii, 384.) 



Au obvious modification permits this instrument to be converted 

 into au electro-dynamometer. A small retangular chamber filled with 

 mercury is placed at the center of a coil of copper wire. The current 

 to be measured passes successively through the coil of wire and the 

 layer of mercury, which communicates laterally with the two branches 

 of a manometer^ and causes a diflerence of level in the mercury columns 

 which is rigorously proportional to the square of the current strength. 

 It may be graduated in absolute measure ; in one of the author's instru- 

 ments a current of 1 C. G. S. unit, or 10 amperes, produces a pressure 

 of 650 dynes, or about 650 milligrams per square centimeter. It may be 

 used for direct or alternating currents, and when once graduated may 

 serve as a standard. {C. B., June, 1884, xcviii, 1534 ; J. Phys., Septem- 

 ber, 188^, II, III,- 387.) 



Several new formf? o f gal vauometer have been described. T. and A. Gray 

 have constructed a very sensitive instrument, consisting of two pairs of 

 coils with hollow cores, arranged so that the axes of each pair are parallel 

 and in a vertical plane, and act on a needle system consisting of two 

 horseshoe magnets of thin steel wire, connected by a very light frame 

 of aluminum, and hung with their planes vertical, so that a horseshoe 

 corresponding to each pair of coils has its poles within the hollow cores. 

 Each pair of coils is carried by a vertical brass plate, the two plates 

 being placed so as to make an angle with one another of about 100°. 

 Certain other instruments are described, in which two perfectly ver- 

 tical and straight needles are used to give an astatic system. {Nature^ 

 March, 1884, xxix. 444.) . , , . 



In D'Arsonval and Deprez's galvanometer the coil is movable and 

 the magnet fixed. A powerful horseshoe magnet placed vertical, with 

 its poles upward, has a rectangular coil suspended between these poles 

 by fine silver wire. A small cylinder of soft iron is supported within 

 the rectangle for the purpose of re-enforcing the magnetic field. A mir- 

 ror above the rectangle enables the deflections to be read. The instru- 

 ment is very dead-beat and will indicate a millionth of an ampere. 

 (Nature, November, 18S4, xxxi, 86.) 



Bosanquet has devised what he calls a standard tension galvanometer, 

 which is a form of tangent galvanometer independent of any process 

 of calibration, and whose accuracy depends only on the usual measures 

 of dimensions and the knowledge of the horizontal component of the 

 earth's magnetism, together with its resistance. {PUl Mag., January, 

 1884, V, XVII, 27.) 



Bottcher's soleuoidal galvanometer consists of a soft iron cylinder i to 

 l^c- in diameter and 20«'" long, suspended by a thread to a spring- 

 balance, so that one half projects from a firmly fixed coil of wire. iPM. 

 Mag., March, 1884, V, xvii, 248.) 



