DETERMINATION OF THE RATE OF VIBRATION OF TUNING-FORKS. 
7 
wave figure is nearly steady : when this is accomplished the observer keeps one wave 
in the field of view by judicious manipulation of the regulating string. He then begins to 
count seconds by the clock and turns the reverser so that the current sent at the 60th 
second will start the electric revolution counter. When the sound of the pen striking 
the drum is heard, he observes what part of a wave is cut by the hah- in the eyepiece, 
and continuing to count the seconds he maintains as far as possible this part of the 
wave near the centre of the field. It is difficult to keep the rotation absolutely 
uniform, but it suffices not to allow the wave to escape from the field of view, or if it 
does, to bring it back as soon as possible. If the observation is to last for one minute 
the reverser is turned in the opposite direction immediately after the first dot has been 
made; but if for a longer period, the contact remains broken by the reverser until after 
the end of the first, second, third, or fourth minute, when the connexions are so made 
that at the end of the next minute the counter is stopped. As soon as the first dot is 
made the screw carrying the electro-magnet is slightly turned so as to shift the 
position of the pen. If at the sound of the marking of the second dot on the drum 
the particular part of the wave which was on the hair at the beginning of the experi¬ 
ment is in another part of the field of view, the alteration of position is estimated as 
accurately as possible in wave-lengths or portions of a wave-length. A correction 
corresponding to this alteration of position is subsequently applied. The only object 
of counting the seconds throughout is to direct the observer’s attention to the approach 
of the minute intervals. The five experiments are made consecutively and without 
stopping the drum. After each experiment the alteration of position of the wave (if 
any), the temperature indicated by the thermometer, and the number shown by the 
revolution counter are noted. When the experiments are finished the dots on the 
drum are marked by consecutive numbers, two belonging to each measurement. A 
long straight-edge is then placed horizontally against the drum and the latter turned 
until the centre of the dot is at the edge of the straight-edge, the number belonging 
to the line of the graduated slip of paper opposite the ruler is then read off. 
A determination made on May 8, 1878, with a 256 fork of Kcenig’s, will best illustrate 
the method of recording and calculating the results, and will also serve to show the 
accuracy of which the method is capable. 
Number of line on circle. 
No. of 1st mark 163 301 
„ 2nd „ 164 199-5 
Duration of experiment, 5 minutes. 
Temperature by thermometer, 16°T C. 
Number shown by revolution counter, 158. 
Number of lines on circle, 486. 
Drum was going a little too fast, the part of the wave under observation had risen 
•5 of a wave-length above the hair. 
