CONTINUOUS ROTATION OF A FRAME. 543 



With a magnet of this latter form the term of correction will be 

 H/V */2 5-6 



-- 



M 3 2000 



= 0*0013 



The degree of magnetisation is often feebler than that which we 

 have assumed, so that the correction may easily amount to several 

 thousandths. The effect of this source of error is to increase the 

 number obtained for the resistance and to diminish the value of the 

 unit. 



Another source of error, the influence of which it is more difficult 

 to eliminate, is due to the induced currents which are developed in 

 the mass of the magnet by the fact of its displacement in a magnetic 

 field, and from variations of the current in the multiplier.* 



1122. CONTINUOUS ROTATION OF A FRAME. The Committee 

 of the British Association utilised the action, which currents induced 

 by the earth in a frame rotating uniformly, exert on a magnetic needle 

 at the centre of the frame. The experiments of the Committee were 

 made in 1863 and 1864.! As some sources of error were observed 

 in this first series of experiments, they were resumed in 1881 with 

 special care by Lord Rayleigh and Professor Schuster,! and in 1882 

 by Lord Rayleigh. 



The rotating frame consists of two identical coils with a space 

 between them in which is placed the apparatus for suspending the 

 wire. Each of the coils has 156 windings of a wire 1-3 7 mm. in 

 diameter, the mean radius being 15 '8 cm. The frame is of copper, 

 but made up of two parts separated by ebonite, so as to offer an 

 obstacle to currents induced in the mass. 



In the first experiments a regulator and a counter were used ; in 

 the latter Lord Rayleigh measured the velocity by a kind. .of phe- 

 nakistoscope. On the axis is a cardboard disc on which are drawn 

 five concentric circles divided into teeth which are alternately black 

 and white, to the number of 60, 32, 24, 20, 16 respectively. This 

 disc is read from a distance by means of a telescope, and through a 

 system of parallel slits supported by one prong of a tuning-fork, 

 which oscillates in front of a fixed screen provided with a system 



* Lord RAYLEIGH. Wiedemanrts Annalen, Vol. xxiv., p. 214. 1885. 

 t British Association Reports for 1863, Newcastle ; for 1864, Bath.. Reprint, 

 pp. 96, 115. 



Lord RAYLEIGH and A. SCHUSTER. Proceedings of the Royal Society, 1881. 

 Lord RAYLEIGH. Phil. Trans. Roy. Soc. 1882. 



