Grasshopper escapement

The term "grasshopper" in this connection, apparently from the kicking action of the pallets, first appears in the Horological Journal in the late 19th century.

A large prize was offered for a solution to the problem and Harrison devoted his life to devising and building highly accurate timekeepers.

Using his clean and absolutely stable grasshopper escapement Harrison began a series of long-term investigations into the performance of clocks,[2] leading to his invention of the gridiron pendulum which counteracted the effects of expansion and contraction with changing temperature.

The performance of his improved clocks in turn gave him an accurate, convenient standard against which to test his marine timekeepers.

This proved to be unreliable,[3] needing repeated attention by which Harrison was inconvenienced, so around 1722 he modified the escapement by putting a hinge in the middle of each arm of the anchor.

In its usual construction this consists of a ratchet wheel sandwiched between, and co-axial with, the first (and slowest-turning) driving gear of the movement and the barrel that the weight (or spring) is attached to.

This is prevented from happening by a pawl fixed to the frame of the clock which engages with teeth cut round the edge of the maintaining wheel.

Once the clock is fully wound, pressure on the key is released and the barrel drives the maintaining wheel and the first gear in the normal way.

In common with other less accurate escapements, the grasshopper pushes the pendulum back and forth throughout its cycle; it is never allowed to swing freely.

Feeling that Harrison's escapement was not well enough known, the clock's grasshopper escapement is exposed on the top of the clock, built in the form of a demonic grasshopper called the "Chronophage" or "time eater", which rhythmically opens and closes its jaws, representing time being devoured.

[10] By eliminating friction as much as possible, such a watch can run for more than a month after rewinding, an exceptional power-reserve.

One of a pair based on the technology of the late regulator clock by John Harrison, it was completed by Charles Frodsham & Company at the request of its owner, Donald Saff.

After successful testing there it was transferred to the Royal Observatory, Greenwich for a detailed assessment of its long-term performance.

[13] This was overseen by the Worshipful Company of Clockmakers and the National Physical Laboratory, whose representatives attached tamper-proof seals to the case of the clock.

At the end of a period of one hundred days, the maximum error had not exceeded five-eighths of a second - no allowance for a steady drift of time (rate) having had to be applied.

The clock remains at the ROG; in 2017 it was moved to the gallery containing the Harrison marine timekeepers where its performance continues to be monitored.