GIO PROF. J. J. THOiMSO:^ AND MR. G. F. C. .SEARLE ON THE RATIO OF THE 
standard fork used by Lord Layleigh. This makes about 128 complete vibrations 
per second. The electrically driven fork maintained another fork whose natural period 
is about half its own. This gave beats with the standard, and by counting the beats 
the speed of the fork through which the stroboscopic disc was observed could be 
determined in terms of tliat of the standard fork. 
Determination of the Speed (f the Standard ForJc. 
It was considered advisable to make a new determination of the standard fork. 
Lord Eayleigh (‘Phil. Trans.,’ 1883, p. 320) found that its speed at t° C was 
128T40 {1 —(t— 16) X -OGOll]. 
We have taken Lord IIayleigh’s value of the temperature coefficient. 
We determined the speed of the standard in the following way. At one end of the 
commutator an endless screw was fixed with a cog-wheel geared into it. The cog¬ 
wheel makes one revolution for 30 made by tlie commutator. A pin fixed to the 
wheel touches a spring once in every revolution, and completes an electric ci)'cuit 
which causes a mark to be made on the tape of the recording apparatus. The 
laboratory clock is also aiTanged so as to complete a circuit once every second. A 
paper tape is pulled along at an approximately uniform rate between guides on a block 
of wood. Two electro-magnets are fixed at right angles to the guides. One of these 
is excited when contact is made by the cog-wheel of the commutator, the other when 
contact is made by the clock. The armatures are kept away from their magnets by 
spiLugs fixed to one end. 
