| 107 
ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 
is brought into communication alternately with Qand x. The disc L m 
was of iron, and its mass helped to steady the motion. On one face of 
_ the dise a series of circles were drawn forming a number of annuli. The 
Pie. 5. 
PE ——————————— eS 
V1 
- successive annuli were divided each into a different number of divisions 
-_ —“— ~- 
by radial marks. Thus in the innermost annulus there were four, on the 
next five, and so on. The disc as it rotated was watched in the usual 
stroboscopic manner through two slits on two pieces of thin metal 
carried by the prongs of a tuning-fork, which made about 64 vibrations 
per second. 
When the frequencies of the disc and of the fork were in certain 
simple ratios to each other, the corresponding pattern on the disc was 
seen in a steady position. The driving pulley of the motor carried a 
second band, which passed over an idle pulley near the observer at the 
tuning-fork, and the speed of the motor, and hence of the dise, was ad- 
justed partly by varying the flow of water, partly by friction on this band, 
until the desired pattern was seen in the steady position. This position 
was easily maintained by varying the friction on the string. The tuning- 
fork drove a second fork an octave above itself in frequency. This fork 
was mounted near the standard fork of the laboratory, and the beats 
between the two were counted. The frequency of the standard fork was 
determined by Professor Thomson and Mr. Searle for their experiments 
on ‘wv,’ recently communicated to the Royal Society. They found that it 
had changed slightly since it was determined by Lord Rayleigh, and give 
as the result of their experiments 
Frequency at temperature ¢t?=128:105 {1—(t—16)-00011}. 
The driven fork was always adjusted to a slightly lower frequency than 
_ that of the standard, so that there were about 20 beats to the minute 
between the two. During each series of observations the beats were 
' Tepeatedly counted, but they rarely varied during the series sufficiently 
ba affect the result. The commutator was designed and partly constructed 
by Mr. Searle, who observed at the tuning-fork throughout. A little 
attention was required to secure good contact between the springs Q, Rk and 
the rotating parts, and also to adjust the brush p, but with moderate care 
in the adjustments the apparatus worked perfectly. 
