216 Prof. S. P. Thompson on a Mode of 



the formation and decomposition of the sulphates is evidently 

 questionable, even from the results he obtained. 



This paper is not of course meant as a criticism on Dr. 

 Wright's work, which I have always admired for its accuracy, 

 but merely gives a new interpretation of certain of his 

 experimental results. 



XXV. Note on a Mode of maintaining Tuning Forks by 

 Electricity. By Prof. 8. P. Thompson*. 



ALL who have worked with self-maintained electric tuning- 

 forks have met with the fact that these instruments as 

 ordinarily arranged give a very irregular note, the pitch of 

 which is continually slightly altering, and even those which 

 are fairly constant in pitch are continually changing the phase 

 of their vibrations. These changes of phase and of pitch 

 render the electrically-sustained tuning-fork as usually con- 

 structed almost useless for acoustic work, and diminish the 

 usefulness of the instrument for chronoscopic and electric 

 applications. They appear to be due to a fact which is toler- 

 ably obvious to any one acquainted with the fundamental 

 principles of the vibrations of elastic bodies, namely, that the 

 impetus given by the electromagnet at each vibration is given 

 at the wrong instant of the motion, namely at some other 

 instant than that during which the fork is passing with maxi- 

 mum velocity through the position of zero displacement. In 

 the older forms of electro-diapason constructed by Fessel and 

 by Koenigt, the electromagnet was of horseshoe form, having 

 poles outside the prongs of the fork ; whilst in the more recent 

 instruments by Koenig the electromagnet is of short cylin- 

 drical form, and placed between the prongs of the fork. The 

 latter arrangement, which is preferable for several reasons 

 electrical and mechanical, seems to have been first suggested 

 by Lord Rayleighf. In the earlier form contact was made 

 by a stylus, carried by the prong of the fork, dipping into the 

 mercury cup : in the later form the stylus usually makes con- 

 tact against a platinum-headed screw, almost exactly as does 

 the interrupter of Wagner so common in electric bells. In 

 either case contact is made at a very brief interval before the 

 prong of the fork reaches its extreme elongation away from 

 the pole of the electromagnet, and is broken at a slightly 

 longer interval after the prong has passed its extreme elonga- 



* Communicated by the Physical Society : read June 26, 1886. 



f Vide Helmholtz, ' Sensations of Tone/ Ellis's edition of 1875, p. 178. 



\ * Theory of Sound,' vol. i. p. 56. 



