PHYSICS. 375 



3. Electrical measurements. 



Stoletow, in a communication to the French Physical Society, has 

 described au apparatus for determining the ratio of the electrostatic to 

 the electromagnetic unit of quantity. It consists of an absolute con- 

 denser, consisting of two metal disks accurately plane, the upi)er fur- 

 nished with a guard-ring. By means of three microscopes, the distance 

 between the plates can be accurately measured. This condenser is 

 charged by a battery, and the discharge current is compared with the 

 constant current jDroduced by the same battery in a circuit of known 

 resistance. In order not to require too large a battery, a series of dis- 

 charges of known number per second is passed throughthe galvanom- 

 eter by means of a commutator, thus producing the eft'ect of a con- 

 stant current and requiring only one Daniell cell. The first results with 

 the apparatus were satisfactory. (J. Pliys., November, 1881, x, 4G8.) 



Eayleigh and Schuster have emi^loyed the original apparatus used 

 by the British Association Committee, for the purpose of redetermining 

 the value of the ohm in absolute measure. Thej' have obtained the value 

 0.9893 earth-quadrants per second, that obtained by Eowland being 

 0.9911. {Proc. Boy. Soc, xcii, 101, 141; Am. J. Scl, December, 1881, III, 

 xxi, 484.) 



Fleming has devised a new form of resistance coil, constructed with 

 a view to avoid the leakage due to condensed moisture on the paraffin 

 insulating the electrodes, and at the same time to facilitate equalization 

 of temperature. The wire is wound bare, each layer being separated 

 from tbe others by strips of ebonite notched to receive the turns, and 

 the whole is inclosed in a brass box screwed together. [Katiire, June, 

 1881, xxiv, 183.) 



Kohlrausch has simplified the apparatus required for his method of 

 measuring resistance, by means of alternating currents. The currents 

 are now produced by an induction coil; and, in place of au electro- 

 dynamometer, the telephone may be used. For liquids, large electrodes 

 of platinized silver are employed. (Wied. Ann., II, xi, G53; J. Fhys., 

 April, 1881, X, 173.) 



Fleming has described a new form of resistance balance, adapted for 

 comparing standard coils, and used by him for measuring coils made of 

 wires of difl'erent alloys. A platinum iridium wire 3^3 inch diameter, 39 

 inches long, and of a total resistance of 0.0512 ohm, is let into the face 

 of a horizontal disk of ebonite, but not flush with the surface ; so that 

 a knife-edge of the same metal carried on an arm moving about the cen- 

 ter of the disk may be put in contact with it. The edge of the disk is 

 graduated into 1,000 parts, and by a vernier on the alidade 0.1 of a 

 division can be measured. This api)aratus is used in a AVheatstone's 

 bridge, two of the other resistances being auxiliary coils of nearly the 

 same resistance each. The two coils to be compared are connected, each 

 with one of the two poles of tlie battery, and each with one of the two 

 ends of the circular Avire. The galvanometer wire connects the x>oint of 



