298 LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE ABSOLUTE 
(equal to the thickness of the disc) was 4^ millims. and allowed sufficient room for 
the contact of the brush though placed tangentially. In this way broader bearing 
surfaces were available, and the small extension of the contact in the direction of the 
axis is unobjectionable, provided everything is arranged symmetrically with respect to 
the middle plane of the disc. 
As will presently appear, the success of the method is independent of any constant 
thermo-electric force at the sliding contact, but it is evident that good readings cannot 
be taken if the thermo-electric force changes its magnitude often and suddenly. It 
was found advisable to renew the amalgamation of the edge at the commencement of 
each clays work. The excess of mercury, if any, attaches itself to the brush, and 
does not appear to render the diameter of the disc uncertain. 
The inner contact was made in a similar manner by a brush pressing against the 
shaft itself at a place a little below that at which the disc was attached. The 
coefficient of induction to be employed in the calculation is the difference between the 
coefficients for the coil and the outer and inner circles of sliding contact respectively, 
but the latter is quite subordinate (§ 25). 
§ 6. The disc was driven by the same water-engine that was employed for the 
revolving coil of former determinations,* the connexion being made by a long cord 
passing round a wooden pulley attached to the lower part of the shaft. To the upper 
face of the disc was cemented a circle of paper on which were marked a series of circles 
of alternately black and white teeth. One observer looking through the prongs of an 
electro-magnetically maintained fork regulated the speed of the disc by application of 
the necessary friction to the driving-cord, which passed through his fingers. When 
one of the series of circles is seen to be stationary, a simple and easily expressed 
relation is established between the frequency of the fork and that of revolution. At 
intervals the number of beats per minute is counted between the notes of a standard 
fork, and (the octave of) the electric fork. There is no difficulty in thus determining 
the speed of rotation to within one part in 10,000. With respect to the absolute 
pitch of the standard fork itself, see the Appendix to this Memoir. 
§ 7. When the disc is caused to rotate, and the galvanometer circuit is closed, a 
deflexion is observed, although the battery which generates the main current is not in 
action. This deflexion is due to two causes—thermo-electric force at the sliding con¬ 
tact, and induction dependent upon the vertical component of the earth’s magnetism. 
Although not a direct source of error, this deflexion is better avoided, both for 
convenience in reading the galvanometer and because it implies the actual passage of a 
not insensible current through the sliding contacts and thus brings into consideration 
the resistance of these contacts. The compensation was effected by the introduction 
of an opposing electromotive force ; for which purpose two terminals of the galvano¬ 
meter circuit J, K, fig. 2, instead of being connected directly, were attached by 
binding screws to two points on a stout copper wire forming part of a circuit which 
* Proc. Roy. Soc., May 5, 1881, p. 112; Pliil.^Trans., Part II., 1882. 
■ I, 
