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Adjusting Bar and It's Use

From The American Horologist and Jeweler magazine, April, 1942

Adjusting Bar and It's Use


It is well known among horologists the advantages a fusee has over the regular going barrel, especially in the older watches.

The first fusee invented, is credited to Jacob Zech of Prague and introduced about 1525. It's purpose was to convert the various forces of the mainspring into a constant driving force. All early English watches and chronometers were equipped with a fusee.

The fusee as constructed today has maintaining power, the principle of this maintaining power is to maintain power to drive the train while the fusee is being turned backwards, or the 'watch being' wound, This style of fusee is called "going Fusee", to distinguish it from the plain fusee as found in the verge watch. The maintaining power was invented by John Harrison of England in 1750, a clever horologist and his improvement is regarded as an outstanding contribution to horology. All marine chronometers employing a fusee as well as high grade weight driven clocks are equipped with maintaining power today. As the mainspring always has more turns of power than are actually used, same means of adjusting or regulating this surplus power in order to obtain constant power throughout the 24 hours, was necessary. This brought about the invention of an adjusting rod.

The adjusting rod consists of long rod with a socket an one end, having a square opening and so made that it can be securely attached to the winding square of the fusee. On this rod is a sliding weight that may be held in place by means of a lock screw or tension spring, holding the weight friction tight. The overall dimensions of the rod varied from eight to twelve inches in length.

To adjust the fusee to it's mainspring, assemble the watch complete with the exception of the fourth wheel, which can be put in place later by removing the small bridge from the lower plate. English watches and chronometer, are all constructed making the third or fourth wheel removable without disturbing other parts of the watch.

Proceed to hook the chain fast to the drum, winding the chain an to. the drum then hooking the chain into the fusee, wedging the third wheel. When the chain is fully wound, set the mainspring up and a half turn. Attach the adjusting rad to the winding square of the fusee, Fig. No. 1. Wind the spring by rotating the rod until all the chain is on the fusee and the stop works have arrested further winding. Hold the movement on edge and slide the weight along the rod until a point is reached at which the weight neutralizes the force of the mainspring so the whole rests in equilibrium when left to itself. Turn the rod backwards one turn at a time and if equilibrium is maintained throughout each test, the fusee is well adjusted. If the weight seems to be too heavy for the lower coils of the mainspring, set the spring up another half turn, or more if necessary to increase the tension. In the converse case, of course, the spring tension must be Jet down. Many times the equipoise cannot be obtained due to a spring being set to much, or possibly, too short or too long. It is profitable and time saving to supply a new mainspring.

To Trace the Form of Fusee 

Having ascertained the distance between plates, mark off on a brass plate the height XY of the cone portion of the fusee, that is to say, of the portion on which the chain is to be coiled. The base of the cone, which is XZ, is the radius to be traversed by the Axis AB at right angles. Divide XY into four equal parts, as there are four turns and draw the parallel lines 3, 5 and 7. Now mount the barrel with it's spring and arbor, between the plates and attach the adjusting Bar, Fig. No.3, to mainspring arbor. Before the spring is brought into action, the adjusting weight "C" must be placed at such a position that the lever rests horizontal. Then wind spring one complete turn, taking note of the required amount of weight necessary to bring the lever into equilibrium. Wind the spring through two turns and add weight until equilibrium is secured, noting the total weight added and so on. When the operation is concluded, there will be five weights in the pan "P". The radii of the fusee will be inversely as these weights and it is easy to determine by a simple proportion by taking the radius XZ as unity, what are the radii C3, D5 and G7 and B9 through the points 9, 7, 5, 3 and X, thus obtained. draw a curve, which will give the form of the fusee for that particular spring.

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