DETERMINATION OF THE RATE OF VIBRATION OF TUNING-FORKS. 
3 
and a piece of gold soldered on to the spring. The pendulum rod thus presses against 
a piece of polished steel and all variable friction is prevented. The figure shows in 
the form of a diagram the arrangement of the connexions. The pendulum is driven by 
four cells of a gravity battery, but as the electro-magnet d has only a small resistance, 
the extra current at first occasionally fused together the contacts of lever a : this was 
prevented by connecting the wires on each side of the contact by a shunt consisting of 
a bobbin of fine wire of about 300 ohms resistance. This plan having, however, proved 
somewhat uncertain in action, the shunt has been replaced by a tube containing two 
platinum plates immersed in very dilute sulphuric acid. 
At the back of the pendulum rod a short pin of chrysolite is fixed to collar Jc. This 
pin, passing under a small steel disk a, Plate 1, fig. 2, raises a thin brass spring- 
fastened to the back of the pendulum case, and makes contact at each oscillation of 
the pendulum with a second spring carrying a platinum pin. The second spring is 
thereby raised sufficiently to make a contact with a third spring, and thus two different 
contacts are made at each second, one preceding the other by a small interval of time. 
The roughening and occasional sticking of the lower contact has been prevented by 
connecting a condenser, or two platinum plates in very dilute sulphuric acid, to the 
springs. The clock, placed at any distance from the pendulum, is actuated by the 
closing of the two lower springs, an electro-magnet raises a lever carrying a catch 
which turns a ratchet wheel with 60 teeth, the wheel being moved by the fall of 
the lever at the breaking of the pendulum contact. Plate 3, fig. 3, shows the arrange¬ 
ment of the minute wheel of the clock and the electric connexions. By an ingenious 
device of Mr. Yeates, who made the clock, the fall of the lever both turns and locks 
the wheel, the end of the catch falling on an inclined screw s which drives the point of 
the catch between the teeth of the wheel. On this minute wheel of the clock is a 
platinum pin a, which presses against a springy once a minute. As this contact might 
possibly be irregular, since it is produced on the breaking of the pendulum contact and 
fall of the clock lever, whereby two elements of uncertainty are introduced, it is 
merely used as a switch, and the principal circuit is closed by the pendulum itself 
when the two upper springs are pressed together. The current then passes through 
the magnet of a relay and closes the circuit required in the tuning-fork apparatus. 
These minute contacts are made with great accuracy, for the upper contact is of very 
short duration, and it is made by the pendulum when moving with nearly its greatest 
velocity. 
The rate of the clock was determined by observations with a small portable transit 
instrument. The arrangements were not as perfect as could be desired, but the time 
was determined from the mean obtained by the observations of at least four stars 
(generally more), and the rates of the clock and chronometer were thus both obtained ; 
moreover the observations were made at such intervals that their inaccuracies did not 
much affect the determination of the rates. The clock was compared daily with the 
chronometer and the irregularities of the former corrected. 
B 2 
