Shortt clocks kept time with two pendulums, a master pendulum swinging in a vacuum tank and a slave pendulum in a separate clock, which was synchronized to the master by an electric circuit and electromagnets.
22.
Since low-pressure is only created on the overrun ( such as when descending hills with a closed throttle ), not over a wide range of situations as in a petrol engine, a vacuum tank is fitted.
23.
Absolute ( reversible ) pendulum gravimeters were replaced in the 1950s by free fall gravimeters, in which a weight is allowed to fall in a vacuum tank and its acceleration is measured by an optical interferometer.
24.
Around 99 % of the energy loss in a freeswinging pendulum is due to air friction, so mounting a pendulum in a vacuum tank can increase the " Q ", and thus the accuracy, by a factor of 100.
25.
He simulated the effect using a " terrella, " a sphere in a vacuum tank to which he directed beams of cathode rays, later identified as electrons, and found they indeed produced a glow in regions around the poles of the terrella.
