446 



On a Uniform Rotation Machine, fyc. [Jan. 18, 



revolve once per second under wide variations of the driving power. 

 The fly-wheel originally consisted of, and now carries, brass disks 

 two feet in diameter, having various numbers of slits cut in them 

 spokewise. Those disks which have been made represent pitches 

 of tenor 0, covering the whole range in practical or theoretical 

 use. 



Tuning forks having any multiple or submultiple of the frequency 

 of the disk employed, present, when examined against an illumi- 

 nated background behind the disk, toothed patterns which remain 

 stationary. If the fork is a little sharp the pattern moves in the 

 opposite direction to the slits, if flat in the same direction. When 

 the machine oscillates about its mean velocity, a fork in tune with 

 the machine presents a pattern which swings backwards and forwards. 



While the machine was yet in an early and imperfect state, it was 

 demonstrated that large tuning forks, when excited electrically with 

 a mercury contact, vary their frequency by change of level of the 

 mercury, within limits which may readily amount to 1 in 200. 



A device due to Lord Rayleigh was employed with the object of 

 extinguishing the oscillations. A ring of metal tubing was filled 

 with water, and mounted as a rim on the disk. But this had practi- 

 cally no effect. An extensive series of experiments was then under- 

 taken, in which large quantities of tubing filled with water were 

 employed. Some slight effect was ultimately obtained, bat nothing 

 useful. The end desired was attained with mercury in india-rubber 

 . tubes. The damping does not as yet amount to a dead beat action, 

 and it is hoped that it may be improved. The damping at present 

 obtainable under practical conditions is reduction of swing to 

 i in 10 s . 



With the machine thus improved, observations have been made of 

 the variations of pitch of a middle C fork, with an electric spring 

 contact, under varying tension. The changes which occur appear to 

 be less than in the case of the mercury contact. 



The experiments that have been made on electromagnetic tuning 

 forks lead to an outline of the theory of the motion in this case, 

 which appears to depend on the time of magnetization of the 

 magnetic system. Although studies have been made on this subject, 

 there is nothing in the shape of a theory depending on absolute 

 measure for guidance in arranging and interpreting experiments. 

 Such a theory is here outlined, and compared with experiment. 



The theory depends on the assumption that the systems dealt with 

 are closed magnetic circuits with small breaks, or cores of solenoids 

 not projecting far from the coils. Under certain limitations the 

 resistance of the magnetized iron can be neglected in comparison with 

 that of the air spaces traversed, according to the values of the 

 permeability of iron furnished by Rowland's experiments. 



