124 Lord Rayleigh and Dr. A. Schuster. [May 5, 



as the fourth term inclusive of the harmonic expansion. Without loss 

 of this property the cylinder may be replaced by four equal line 

 magnets, coinciding with four symmetrically situated generating lines. 

 Thus, if we make a compound magnet by placing four equal thin 

 magnets along the parallel edges of a cube, the length of the magnets 

 should be \/S times the side of the cube. This is on the supposition 

 that the thin magnets are uniformly magnetised, as is never the case 

 in practice. To allow for the distance between the poles and the ends 

 of the bars, we may take the length of the bars 2*3 times the side of 

 the cube. 



With the new apparatus, and with the precautions pointed out by 

 experience, we hope to arrive at very accurate results, competing on at 

 least equal terms with those obtained by other methods. Most of the 

 determinations hitherto made depend upon the use of a ballistic gal- 

 vanometer, and the element of time is introduced as the time of swing 

 of the galvanometer needle. There is no reason to doubt that very 

 good results may be thus obtained, but it is, to say the least, satisfac- 

 tory to have them confirmed by a method in which the element of time 

 enters in a wholly different manner. 



Part II. — By Arthur Schuster. 



Adjustment of the Instruments and Determination of Constants. 



The only adjustments to be made consist in — 



1. The levelling of the coil. 



2. The suspension of the magnet in the centre of the coil. 



3. The proper disposition of the scale and telescope by means of 

 which the angles of deflection are read off. 



Level. — The first of these presents no difficulty, and any small error 

 can be easily taken account of in the calculations. It was found 

 that the upper end of the axis of rotation was inclined towards the 

 north by an angle of "0003 circular measure. Hence, as has already 

 been explained (page 107) we must in the reductions write through- 

 out G (1 + -0003 tan I) or 1-0008 G for G. This correction is small, 

 but a little uncertain, as the coil was not very steadily fixed in its 

 bearings, and small variations in the inclination of the axis could be 

 produced by slightly pressing on one side or the other of the coil. 

 When left to itself the coil seemed, however, very nearly to return to 

 the same position. 



The Magnet. — The magnet, which was suspended in the centre of the 

 coil, consisted of four separate magnetised needles, each about 

 0'5 centim. long. These were mounted on four parallel edges of a 

 small cube of cork. A needle attached to the back of the mirror went 

 through a small hole in the cork, and was kept in its place by means 



