No, a Foucault pendulum is not a good watch. The rotation period
of the plane of oscillation varies too much. But still it can be used to give an
approximate time. The period is among other things sensitive for possible
asymmetries in the suspension and if the pendulum is exposed for draught.
Theoretically, the period of the rotation is
T = 24/sin λ, where λ = the latitude of the
place
In Motala, situated on a latitude 58° 33´ North, the period is T = 28 h 3 min
We made 10 measurements of the period for a complete rotation during a 14-day period. For our pendulum we got a mean 26 h 59 minutes, with a standard deviation about 17 minutes.
The size of the fictitious coriolis force is given by
Fcor= -2mωv, where
m = the mass of the bob, v = the velocity of the bob and ω is the
angular velocity of Earth (ω = 7,292.10-5 rad/sec ).
The force is biggest when the velocity of the pendulum is biggest, that is
when the bob is in its lowest position.
For our pendulum the difference in level between the highest and the bottom position is about 1,33 cm. Using mgh = mv2/2 gives the velocity 0,51 m/sec at the very bottom position of the bob.
Fcor= -2mωv then gives the coriolis force about 4,8.10-4 N.
Thus the force acting on the bob is very small. It is of the same magnitude as the force of gravity on a stamp. But as many small, small alterations of direction are added by every oscillation the effect is a continuous rotation.
