VALUE OE THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
317 
through, the coils of small electromagnets, maintained in vibration not only the inter¬ 
rupter fork itself, but also a second fork of pitch about 128. After the apparatus has 
been a short time in operation, the vibrations of the second fork are exactly four times 
as quick as those of the first, independently of any precise tuning ; and they give rise to 
audible beats when the standard fork is simultaneously excited. In the presence of 
extraneous noises the observation of the beats is much facilitated by the use of 
resonators, with one of which the ear may be connected by an indiarubber tube. The 
object to be aimed at is to make the intensities of the two sounds (as they reach the 
ear) very nearly equal. The moment of antagonism is then marked by a well defined 
silence, whose occurrence can be timed to within a second, although the whole duration 
of the beat may be 20 seconds or more. Without fresh bowing of the standard, the 
silences can be observed satisfactorily for at least a minute. 
In the first determination the comparison between the fork of frequency 32, and the 
pendulum of the clock was made directly. The observer, looking over a plate carried 
by the upper prong of the fork, obtained 32 views per second, i.e., 64 views of the 
pendulum in one complete vibration. The immediate subject of observation is a 
silvered bead attached to the bottom of the pendulum, upon which as it passes the 
position of equilibrium the light of a paraffin lamp is concentrated. Close in front of 
the pendulum is placed a screen perforated by a somewhat narrow vertical slit. If 
the period of the pendulum were a precise multiple of that of the fork, the flash of 
light which to ordinary observation would be visible at each passage, would either be 
visible, or be obscured, in a permanent manner. If, as in practice, the coincidence 
be not perfect, the flashes appear and disappear in a regular cycle, whose period is 
the time in which the fork gains (or loses) one complete vibration. This period 
can be determined with any degree of precision by a sufficient prolongation of the 
observations. 
On account of the large number of views per second, the interval between successive 
visible positions of the bead, even when it is moving with maximum velocity, is rather 
small; and thus the adjustment of the apparatus is somewhat delicate. ” c In order to 
meet this objection, a modification has been introduced, which must now be explained.! 
A few years ago it was shown almost simultaneously by La Cour and by Lord 
Rayleigh, that an electromagnetic engine could be accurately governed by an inter¬ 
rupter-fork. The construction (fig. 6) which has been found most suitable is similar 
to that of Froment’s engine. A horizontal shaft revolving upon steel points carries a 
* In the earliest use of this method (“ Nature,” xvii., p. 12, 1877) the break-fork had a frequency of 
about 13, and no difficulty of this kind was experienced. 
t [July, 1883.—It should be stated, however, that the wheel may easily be dispensed with, if proper 
care be taken in the illumination of the bead and in the management of the fork. The vibration should 
be vigorous, and the screens so arranged that the view past the fork at the moment of greatest elongation 
should be of short duration. Determinations by this method (without the wheel) have often been made 
successfully by students in the Cavendish Laboratory.] 
! 
