The Mariner's Chronometer

A few weeks ago I received a chronometer which seems to have been heavily damaged by dropping, possibly while out of its bowl . The upper balance pivot was broken, the upper jewel of the third wheel was absent, or rather, it was present in many tiny pieces, and there were burrs raised on a few adjacent teeth of the great wheel. The stop bar of the stop work was stuck and the maintaining power did not work. The hands were absent and so was the fusee chain.

I have covered re-pivoting in posts 6 and 7 and making hole stones in post 26, though when there is no end stone to take into account, the process of making one is a rather simple turning operation. A little careful filing removed the burrs on the great wheel. Dismantling the stop work and maintaining power revealed dried-up grease, so that merely cleaning the parts of all old lubricant and replacing it with new brought these back to life. That left the hands to make and the chain to replace. I will cover making hands in a later post.

To a mariner, “cable” used to mean “A large rope by means of which the Ship is secured to the Anchor.” When it became possible in the nineteenth century to make large chains economically, they began to talk of “chain cable”. While the weight lines in clocks were of gut, this presumably was not strong enough to use as the link between spring barrel and fusee, so chain was used. It may well be that the technology of the eighteenth century was not equal to making wire lines that were both flexible and strong, and since chronometer makers were nothing if not conservative, the use of chain continued to the end of chronometer manufacture. These chains, which look rather like miniature cycle chains, were made by specialists and the chronometer repairer limited himself to repairing a broken link, usually by simply removing it.

Such books as there are that tell of chronometer repair are silent on what to do when the chain is completely absent. I found that I could buy a replacement chain for a trifling US$190, but as readers will perhaps by now have gathered, I am no spendthrift, and I began looking for an alternative to chain. The problem resolved itself into several requirements. The replacement has to be strong enough for its task; it has to be flexible enough; and it has to be possible to attach it to the barrel and fusee without special techniques. It also has to fit in the pre-existing groove of the fusee. These groves are always flat-bottomed so that conversion to a round cable is possible.

Less than a kilometre from my house is a famous game fishing club from which tormenters of large fish set sail. It is considered unsporting of the fish to bite through the trace of the hook and so it is often nowadays made of steel wire. A visit to a local supplier of fishing tackle provided me with a metre of stainless steel wire 0.8 mm in diameter for the sum of NZ$1.20 and I thought my problems were at least partly solved.

I carved hooks out of some scrap roofing iron using a piercing saw and files, and immediately encountered some problems: I could not solder the wire to the hooks, it was not flexible enough to allow me to pass it through a hole and bind it back on itself and when I made a little brass bush and squeezed it on to the end of the wire, it projected enough to foul either the top plate or the ratchet wheel of the maintaining power or the centre wheel arbor. By creative bodging I managed to overcome this, only to find that the wire was almost too stiff to be able to fit it. I then recalled that many years ago a kind person had given me a box of bits and pieces that he had bought at a deceased clock maker’s clearance auction with no clear idea what to do with them. Among the bits and pieces were several pieces of flexible wire and one length of the correct diameter to fit between the cheeks of the fusee groove. It appeared to be of multi-stranded steel, was very flexible; and it could be doubled back on itself with relative ease.

A test of its solderability revealed that it was plastic-coated and when I burned off the coating and scraped the wire clean, its fine structure could be revealed and contrasted with the relatively coarse structure of the stainless steel trace wire ( middle, Figure 1). The figure also shows how I at first anchored the trace wire to a hook with a bush.

Figure 1: Stainless steel trace (left and centre) and multi-stranded flexible steel (right)

Attaching the cable to the barrel with a normal-shaped hook was relatively easy, using techniques well known to a mariner setting up standing rigging. In my case, after doubling back the wire on itself I held the bight in a small vice while binding it with some fine nichrome wire that I had bought for another purpose. As Figure 2 shows, the free end  just clears the top plate.

Figure 2: Attachment of cable to the barrel hook.

Attaching the other end to the fusee hook required some modification to the normal shape of the hook, as otherwise the standing end of the wire would not lead into the beginning of the fusee groove. Thus, I made the hook deeper than usual so that the end of the hook could be twisted through 90 degrees and bring the cable in line with the fusee groove (Figure 3).

Figure 3: Attachment of cable to modified fusee hook.

I have not been able to find a supply of similar wire. Phosphor bronze wire 1.5 mm in diameter is available but is too big. I tried unravelling  a sample of the wire and using only three of the five strands, but it stubbornly refused to be pulled straight. As it was hard drawn, I tried annealing it in a gas flame while pulling on it, but it broke under a very moderate stress. Clock fusee chain is too wide and pocket watch chain is too short.so I am very glad to have been able to find just the right diameter.

After a thorough overhaul and repair, the chronometer sprang into life with a very strong balance motion. I have gradually regulated it  over three days and it is currently losing at the rate of 1.3 seconds a day. I hope that with a little more tweaking it will do a little better.

If any reader knows where to obtain flexible, multi-stranded steel or phosphor-bronze wire of between 0.8 and 1.0 mm in diameter, I would be glad to hear of it.