employed for determining the Ohm. 263 



unit time by known methods offers no special difficulty; nor 

 does the maintaining of a constant velocity of rotation by 

 mechanical means*. 



7. The adjustment of the magnet in the centre of the rota- 

 ting coil should also not be difficult to effect. Moreover a 

 small deviation from exact adjustment causes no important 

 error. 



8. The moment of the magnet may be determined by vibra- 

 tion-experiments after determining its moment of inertia and 

 the horizonal component of the earth's magnetism; or it may 

 be determined from experiments on deflection. If, in order 

 to render the inductive action of the magnet on the rotating 

 coil imperceptible, we employ magnets of very small moment, 

 then these methods offer many sources of error, on account of 

 the very perceptible influence of the friction of the air in the 

 vibration-experiments, or of the small distance at which the 

 deflecting magnet must be placed. 



If the moment of the magnet is very small, it and the dis- 

 tribution of magnetism in the magnet (which is very difficult 

 to determine) have both very small effect upon the results. 

 If the poles of the magnet are at a distance from the cen- 

 tral point in the median plane of the coils which is less than 

 \ of their radius, and if the distance of the poles from the 

 plane is not greater than 0*84 of the length of the magnet, 

 then the force exerted upon it by a current in the coil is con- 

 stant to within O'OOOo, under conditions otherwise similar, 

 up to a deflection of 56°. 



9. The adjustment of the telescope and scale required for 

 reading-off the position of the magnet, and the correction of 

 the readings, may be made in the usual way; and the divisions 

 of the scale must be compared with an accurate measure. The 

 accurate measurement of the distance of the scale from the 

 central point about which rotation of the magnet takes place, 

 or from the reflecting surface of the mirror, offers a certain 

 amount of difficulty. 



10. The force of torsion of the suspension-thread of the 

 magnet may easily be compared with the directive force of 

 the magnet by turning the thread fastened to a torsion-circle 

 through a certain angle, and observing the deviation of the 

 magnet. Unavoidable changes of considerable magnitude 

 may result from the variable moisture of the air, if weak mag- 

 nets are employed. 



11. We have, further, to inquire what influence currents of 

 air and the vibration of the apparatus caused by the motion 



* Compare the ingenious arrangement adopted by Lord Rayleigh, 

 loc. eit. 



