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electric clocks

 A history of the evolution of electric clocks

H. Shortt's free pendulum

We have now come to the final stage of the development of electric clocks. William Hamilton Shortt coupled two Synchronome clocks so that one, the slave, would accomplish the release of the gravity arm of the other, the master. Using his “hit and miss” system he was able to have these pendulums “communicate” with each other.

William Hamilton Shortt (1881-1971)
In 1921 William Hamilton Shortt, a British railway engineer and director of the Synchronome Co Ltd, succeeded in devising a system to keep two pendulums in precise sympathy (patent no.187814).

Shortt A leaf spring is fixed to the shaft of the slave pendulum and an armature is pulled down by an electro-magnet receiving its synchronizing signal from the master clock. The slave pendulum is given a small permanent losing rate so that, when its spring is late in arriving at the tip of the armature, it is caught and deflected, thereby quickening the semi-vibration of the pendulum, since the spring adds to the force of gravity.

If the spring is early in arriving at the tip of the armature it will miss catching the armature, hence the term “hit-and-miss”.

The tension of the spring is such that the advance effected is twice the loss (due to the losing rate of the pendulum) during the interval between two impulses, with the result that the engagement takes place approximately at every second impulse.

On the right-hand side we see an ordinary Synchronome clock (the slave) to which a leaf spring is added and the "hit and miss" slave synchronizer.

On the left-hand side we see the free pendulum (the master) placed in a vacuum tank. There is no count-wheel here and only an electro-magnet has been added to operate the catch of its gravity arm.

Experiments carried out by Shortt showed that if the vacuum of the tank is kept at 3 cm of mercury the energy consumed by flexing the suspension spring equals the energy consumed by deflecting the remaining air molecules; thus a higher vacuum is not required.

When at half-minute intervals vane a fitted to count-wheel b releases gravity arm c, the slave pendulum will receive its impulse. After completing to give its impulse, the gravity arm touches contact screw d of armature e. Electro-magnets f are now energized and their armature resets the gravity arm. At the same time electro-magnet g of the free pendulum is energized and releases gravity arm h of the free pendulum that is on its way to zero on its excursion to the left. Jewel j, fitted to the end of this gravity arm, now falls onto a little wheel fitted to a bracket supported by the pendulum. And so the impulse begins when jewel j runs down the periphery of the wheel and terminates when it drops of the wheel.

The gravity arm then falls upon the wing of catch k and releases switch arm m, which in falling replaces the gravity arm onto its catch. At the end of its fall contact is made with armature p and switch arm m is reset by electro-magnet r.

When magnet r is energized, magnets s and t are also energized operating the hands of the dial and pulling down armature v of the “hit and miss” synchronizer.

The considerable time that elapses between the release of the gravity arm h and the operation of the remontoire replacing switching arm m, can now be realized. It amounts to about 0.7 - 0.8 of a second, during which period the slave pendulum, which was a little beyond zero on its excursion to the right, completes its swing and returns to zero in time for the act of comparison which determines whether or not correction is required.

When the slave pendulum is late, leaf spring z will be caught and deflected by armature v and thus quickening the semi-vibration of the pendulum. When the slave pendulum is early, the armature will miss catching the leaf spring and nothing will happen.

Disregarding the energy required to flex the suspension spring and deflecting the air molecules, the free pendulum is swinging freely and has no work to do of any kind, none what so ever! The only disturbance to its freedom is the acceptance of an impulse from its gravity arm at half-minute intervals.

The introduction of the Synchronome-Shortt Free Pendulum was a great step forward and about one hundred of these clocks were built, the first Synchronome-Shortt Free Pendulum being installed in the Edinburgh Observatory. Suddenly the accuracy of time measurements was increased to about one millisecond a day and it was not long before astronomical observatories all over the world made use of this system.

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Electricity & Magnetism
Electricity & Horology
   A.Electrostatic clocks
   B.The first inventors
   C.Independency of battery
   D.Reliability of contact making
   F.Count-wheel and impulse
   G.The first free pendulum
   H.Shortt's free pendulum
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page 11.
master and slave