628 SCIENTIFIC RECORD FOR 1885. 



be brought very uear the center of the coils. Without the exterior 

 magnet, O-l™"^ deflection at a scale distance of 2-7 meters corresponds 

 to 54 X 10-'" ampere; with this magnet, to 12 x 10"'" ampere. With 

 a German-silver and iron couple a difference of 10° between the junc- 

 tions gave a deflection of 120°'"' through 1,000 ohms. {Wied. Arm., 

 XXIII, 677 5 Am. J. Sci., February, 1885, III, xxix, 167.) 



Anthony has devised a large tangent galvanometer for the laboratory 

 of Cornell University as a standard instrument for the measurement of 

 heavy currents and for the direct calibration of commercial measuring 

 apparatus. It has four circles, two of which are 2 meters in diameter 

 and two are 1-6 meters, mounted on the plan of Von Helmholtz at dis 

 tances apart equal to their radii, and made of rods of copper 0-75 inch 

 in diameter. The needle is suspended by a silk fiber and is inclosed in a 

 mass of copper, which serves as an effectual damper and enables readings 

 to be made very rapidly. A special arrangement of mirrors and tele- 

 scopes permits the reading of the deflections in angular measure on a 

 circle 50 inches in diameter to within 0-3 of a minute of arc. The cop- 

 per conductors are mounted on a brass framework accurately turned 

 and adjusted and the dimensions are all known within one five- thou- 

 sandth part. For the measurement of currents there are two cir(;les, each 

 1-5 meters in diameter and each having two conductors, together com- 

 prising seventy-two turns of Ko. 12 copper wire. {Electrician and Elec- 

 trical Engineer, October, 1885, iv, 372 ; Nature, October, 1885, xxxii, 634.) 



Mather has suggested the calibration of a galvanometer by a constant 

 current as follows : A current is passed through its coils, and the instru- 

 ment is turned through any angle and the deflection 6 noted. The 

 current is broken, and the needle swings back into the meridian, pass- 

 ing through an angle 8. This operation is repeated with the sauie cur- 

 rent, the galvanometer being in various positions; and a curve is drawn 

 showing the relation between sin B -^ sin ^5 and the corresponding values 

 of 6. When now the instrument is used in its normal position it is 

 obvious that a current producing a deflection d of the needle is propor- 

 tional to the value of sin ^-^siu <J corresponding to 6 obtained in the 

 calibration experiment ; and this value can be read off" directly from the 

 curve. {Nature, December, 1885, xxxiii, 166.) 



Trowbridge, while in general preferring the electro-dynamometer in 

 the form devised by him to his cosine galvanometer (described in 1871) 

 for the measurement of strong currents, yet has suggested a method of 

 using the latter instrument which removes most of the objections. The 

 galvanometer is so mounted that its compass is at the center of a large 

 circle of wire the plane of which is vertical and in the plane of the 

 needle; When the strong current is passed through the large vertical 

 coil the arrangement acts as a tangent galvanometer. The movable coil 

 of the cosine galvanometer is then connected with a Daniell cell of 

 known electro-motive force, and in the same circuit a resistance is placed 

 so huge that the battery resistance may be neglected, and, having joined 



