PHYSICS. 627 



leut to the resistance of a mercury column having a section of 1 square 

 millimeter and a length of 105*98 centimeters at 0° centigrade. {Phil. 

 Mag., November, 1885, V, xx, 417.) 



Wild gives from his measurements the number 0*94315 ohm for the 

 value of 1 Siemens unit, and therefore gives lOG-027 centimeters as the 

 length of the mercury column representing the ohm at 0°. {Am. J. Sci., 

 February, 1885, III, xxix, 168.) 



Because of the importance of having as little heating as possible in 

 all instruments for electric measurement, the question has arisen whether 

 the coils of such instruments should be made of German-silver wire or 

 of copper wire or partly of both, and how the diameters of the wire should 

 vary in different parts of the coil. Ayrton and Perry have investigated 

 the conditions that make this heating error a minimum with cylindrical 

 coils of given internal and external radii, and have reached the conclu- 

 sion that the wire should be of copper and that the increase of cross- 

 section proceeding from the center should be x=Xo r^-*. Other points in 

 connection with these instruments were discussed. {Nature, July, 1885, 

 xxxii, 215.) 



Fleming has constructed a standard Daniellcell, consisting of a U tube, 

 in the two limbs of which are the two solutions of copper sulj^hate and 

 zinc sulphate of the same specific gravity. Electrodes made of freshly 

 electro deposited copper and pure zinc that has been twice distilled dip 

 into the two limbs. The electro-motive force of this cell is 1.102 volts, 

 and the variation with temperature is practically nil. The various con- 

 ditions affecting the electro-motive force of this cell were carefully studied. 

 {Phil. Mag., August, 1885, V, xx, 126; Nature, July, 1885, xxxii, 263.) 



Hesehus has designed an amperemeter founded on the iihenomenon 

 of Peltier. A thermo-electric battery of 12 iron-German silver elements, 

 in the form of wires 2*3'"™ in diameter, is so arranged that the opposite 

 junctions are contained in two vessels which form the reservoirs of a 

 differential air thermometer. A current passed through the battery heats 

 the even junctions and cools the uneven ones, or vice versa; and this 

 causes a change of level in the thermometer proportional to the inten- 

 sity of the current, eliminating the heating of the conductors. One 

 division on the scale of his apparatus corresponds to 0*66 ampere. {J. 

 Soe. Phys. Chim. Busse, xvi, 452 ; J. Phys., December, L885, II, iv, 587.) 



Rosenthal has devised a galvanometer of great range and great sensi- 

 tiveness, the needle of which is a horseshoe magnet suspended by a 

 long fiber attached to its neutral point. The poles of the magnet are 

 provided with horizontal pole pieces which are quadravital arcs of a 

 circle the center of which lies in the axis of suspension of the horseshoe. 

 These pole i)ieces can play within the axis of two galvanometer bobbins 

 placed on opposite sides of the vertical suspension plane when this 

 plane coincides with the magnetic meridian. When an electric current 

 passes through the coils these pole pieces are respectively drawn in or 

 repelled by the two bobbins. In this way the poles of the magnet can 



