428 EEPOKT — 1881. 



frame in wliich it was mounted, were in the main copied from those used 

 in the experiments of the former Committee, but both riug and frame 

 were made considerably stouter in the metal, and the ring had only one 

 groove instead of two. The upper and lower halves were insulated from 

 each other, to prevent the formation of induced currents. 



To determine the area A, we have A = ««r/47r, where n is the 

 number of turns of wire on the coil and vi the circumference of the mean 

 layer. The value of m was ascertained by measuring with a steel tape 

 the circumference of the groove in which the coil was wound, as well as 

 the circumference after each successive Inyer of wire had been put on. 

 The mean of all these measures, corrected for the thickness of the tajse, 

 about O'Ol cm., was taken as the value of m. In order to guard against 

 accidental error, the separate measurements of the circumference were 

 combined in pairs, thus i/io + i»,„ «ii + «'„-i, . . . the suffixes denoting 

 the numbers of layers of wire which had been wrapped on when the 

 several measurements were made ; these sums, which ought to be con- 

 stant, varied between 193'25 cm. and 193'50 cm., the number of layei's 

 being 32. In this way the circumference of the meaii layer was found 

 to be 96'669 cm. which gives for the area enclosed by it 743"G6 sq. cm. 

 Each layer of wire contained 30 turns, and therefore n = 30 x 32 = 960, 

 and the total effective area of the coil was A = 960 X 743'66 = 713914 

 sq. cm. 



The tangent-galvanometer had two equal parallel coils, of approxi- 

 mately square section, placed at a distance apart nearly equal to their 

 mean radius, which was about 18'25 cm. Bach coil consisted of 22 layers 

 of 20 turns each ; the galvanometer had thus altogether 880 turns of 

 wire. The needle consisted of three short bits of hardened and magnetised 

 watch-spring, fastened one above another at the back of a light plane- 

 glass mirror. The deflections were read upon a straight glass scale, 

 divided at the back into millimetres. The distance from mirror to scale 

 was 137"25 cm., of which about "45 cm. was occupied by glass ; the 

 optical distance was thei'efore taken as 136'95 cm. The galvanometer- 

 constant r was calculated by the formula 



T=^(b, log, "' + ^_;!l!+ F -&. log, ^iJl^^J^L^M) 



where » is the number of turns of wire (880) in the two rings taken 

 together, s and s' the areas of the cross-sections of the two coils, «! the 

 external radius of each coil, a2 ^^^ internal radius, i»i the half-distance 

 measured parallel to the axis between the outer surfaces of the coils, and 

 h.2 the half-distance J^etween their inner surfaces. The numerical values 

 were a^ = 18-945 for one coil, = 18-9.53 for the other; a.2 = 17-518 and 

 17-524; hi = 9851, and h = 8-429, all in centimetres. The values for 

 «! and also for a.^ being so nearly alike for the two coils, the means a^ = 

 18-949 and a, = 17-521 were used in the calculation of F. The numerical 

 value of r was thus found to be 1/0-004618, so that the absolute 

 strength of a current measured upon this galvanometer is 



0-004618 H tan 8. 



The commutator of the revolving coil consisted of a cylindrical piece of 

 ivory about 7'6 cm. in diameter, with two pieces of platinum let in upon 

 opposite sides. One end of the wire was fastened to one of these platinum 

 pieces and the other end to the other piece ; and contact with the external 



