628 SCIENTIFIC RECORD FOR 1885. 



be brought very uear the center of the coils. Without tljc 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. Atm., 

 XXIII, 677; 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 eftectual dam{)er 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 circles, each 

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

 prising seventy-two turns of No. 12 copper wire. (Electrician and Elec- 

 trical Engineer, October, 1885, iv, 372 ; JS/ature, 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 S. This operation is repeated with the same cur- 

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

 showing the relation between sin 6 -^ sin d 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 8 of the needle is propor- 

 tional to the value of sin 6'-;-siu (5 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 hioi to his cosine galvanometer (described in 1871) 

 for the measurement of strong currents, yet has suggested a n)ethod 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 galv^anometer is then connected with a Daniel! cell of 

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

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



