Chapter XI: The fusee

111. In the age of the recoil escapement the invention of the fusee was unquestionably the most important step for achieving good time keeping by watches. The old verge watch was so influenced by changes in mainspring power that it hardly deserved the title of a timekeeper unless it was provided with a mechanism to counteract these irregularities. The verge escapement was replaced by the resting escapements, particularly the cylinder escapement. One of the main characteristics of the latter is that the lock and lift take place at the same distance from the centre of the balance; therefore the locking friction is considerable and happens during the greater part of the oscillation. These circumstances mean that an increase of locking friction goes hand in hand with an increase in mainspring power, and this friction works in a correcting way if the escapement is well designed. Then irregularities in power affect the rate by a surprisingly small amount. The duplex escapement works in a similar way; while the free escapements, which do not have a correcting friction, achieve their high independence from power variations only by a prudent adjustment of the balance spring.

112. From the time when these facts were clearly understood the leading watchmakers in the various centres of watch production followed very different approaches. The French and Swiss, with their practical gift, took immediate advantage of this changed state of affairs and simplified the work by eliminating the fusee and its associated parts. This step, together with some other circumstances, formed the basis on which Swiss production greatly expanded, because it was able to manufacture a cheap watch of convenient and even delicate design which, nevertheless, was sufficiently accurate for civil use.

113. In contrast, the English retained the fusee in these greatly changed circumstances; and even though an equal number of voices in favour of the going barrel have arisen amongst them of late, the majority hold to the faith that the fusee is an indispensable characteristic of a good English watch. The consequence of this conservative attitude is a well deserved superiority of time keeping in their better watches, but a gradual decrease in demand for their lesser grades, which indeed have slowly ceased to be marketable.

114. These are the practical and commercial results of the retention and omission of the fusee in modern watches, as shown by experience in the old production countries. It is surprising that the famous invention of Graham, the cylinder escapement, was little used by his compatriots; they rejected the idea merely because the nature of the new escapement makes it necessary, or at least desirable, to change the layout the movement. The Swiss, by adopting the latter approach and introducing a far-reaching division of work, succeeded in manufacturing a watch with good time measurement, of elegant style and size, and at a marketable price; and so their production greatly increased.

115. There is no doubt that the fusee, with its compensatory effect, secures a higher uniformity of rate in watches of the best class; but the degree of superiority which is achieved is greatly overrated, and for ordinary use there is no difference of practical importance between the rate of a fusee watch and that of a going barrel watch. Even if, between the first 6 and the last 6 hours of the mainspring, there was a difference of 10 or 20 seconds in the rate of a going barrel watch (which is a very much larger deviation than can occur from this cause in a good watch), then this is no obstacle to the general reliability of the watch; because the error repeats itself every 24 hours, and it would only be necessary to wind the watch in the most regular way possible.

116. The use of the going barrel permits a larger train, a roomier barrel and a less restrained layout of the moving parts. It avoids the frictional resistance of the two large pivots of the fusee, and has the great advantage of not being exposed to the many accidents involving fusee work, not only the danger of breaking a mainspring, but also that of the chain breaking. Work with a going barrel, if it is designed in an appropriate way so that it has a long and thin mainspring, can have a total development of at least six turns with the middle ones reserved for running, and such a mainspring is not as prone to breaking as the thick and short spring used for fusee work.

117. However, the greatest advantage of all is that going barrel work, with its greater abundance of motive power, is more suitable than fusee work for fast oscillations of 18,000 per hour. This fast oscillation makes a watch more suitable for exact time measurement, particularly if it is carried by a person who drives and rides, or it is exposed to continual outside vibrations in other ways. It is apparent that the much greater momentum of a quickly oscillating balance will be far less subject to the influence of such disturbances than another balance which oscillates a fifth more slowly. This increased activity of the mechanism, with 3,600 more oscillations in an hour, will naturally require a greater motive power, and in this situation fusee work has no advantage, unless it has a greater height and diameter, or unless it has a very light balance.

118. The above points can be combined into the following conclusion: the use of a fusee is recommended for all watches from which the most exact time measurement is expected. However, the going barrel should be used for all watches which do not belong to this class, and particularly for the use of people who require their watch to be as little affected by disturbances as possible; such as travellers, soldiers and so on.

119. This criterion was probably understood by the first watch manufacturers in the United States when they dropped the fusee from their movements and, in my view, herein lies a very substantial part of their success.

120. Having stated the situation in which the application of the fusee is useful, it is not redundant to say something about the best design for strong and well proportioned fusee work. Here I must say that I do not think the old English fusee work deserves to be regarded as the perfect arrangement, because it cannot use a mainspring whose width is in relationship to the height of the frame. This, as I will prove by designs and numbers, is to be attributed mainly to the fact that the centre wheel is not favourably positioned in such movements. When I worked in London, I had a number of conversations on this point with very good watchmakers who, in the most determined way, advised me against any attempt to change the construction of fusee work. I sketched a design in which I could not see any mechanical faults and which was certainly complete, but at that time I lacked the means to implement it. However I found an opportunity later and the test confirmed my first opinions totally. I give here the design and description of my fusee work with numbers showing its comparative advantage over English work, in hope that it might be useful to the reader.

        As will be easily seen, the greatest change in this work is to transfer the centre wheel from its usual place under the barrel to the opposite side of the frame, over the barrel and fusee. The centre wheel can be sunk very conveniently into the upper plate so that it lies flat with the lower surface of the plate; then the fusee can come close to the upper plate as in English work. However, the barrel cannot go through the upper plate, as is usually the case in English work, but it can reach down almost to the dial, with the exception of the thickness of its lower bridge. In English work the centre wheel is an absolute obstacle to the greater height which could be made available to the fusee and barrel, and the whole height of the frame between the centre wheel and dial is completely lost for these important parts.

Figure 30.
Improved arrangement of fusee work (view from the upper plate).

Figure. 31.
         Improved arrangement of fusee work (view under dial).

         In order to show more clearly the advantages which follow from this layout, I give the following comparison.

        I have a good English three-quarter plate movement; diameter 44 mm, overall height of the frame 7.2 mm, the height of the fusee 3.2 mm and the height of the Barrel 2.65 mm.

        My work with the changed layout has a diameter of 46 mm, its height is also 7.2 mm, the height of the fusee 3.8 mm and that of the Barrel 3.9 mm.

        Since the height of the frames are the same in both cases, the apparent advantage is clearly due to my arrangement.                

Height of fusee Height of barrel
In my fusee work 3.80 mm 3.90 mm
In English fusee work 3.20 mm 2.65 mm
Difference in favour of the first 0.60 mm 1.25 mm

        Compared with the English work with the centre wheel over the fusee, my design increases the height of the fusee by 19 percent and the width the mainspring by 47 percent.

        This latter is an increase of almost one half and I believe it is a very substantial improvement in fusee work. From the description of my design the reader can see that this gain was not bought by losses in the durability of the other parts of the movement.

        The 3rd wheel in work of this kind must go on the dial side of the pillar plate under the fusee wheel, in all other details there is no difference from the usual arrangement of the parts.

121. The positions of the fusee and barrel is not correct in English work and they should be turned around. This latter position of the fusee saves a large amount of friction on the pivot without causing any loss or disadvantage.

        The pressure on the fusee pivot in English work is greatly increased by this constructional defect. Fig. 32 shows the positions of the fusee and centre wheels. To determine the pressure on the pivot we assume the point of contact between the wheel and pinion at f is the fulcrum of a lever, at whose other end g the force is transferred by the chain. It does not require a proof that the pressure on the fusee pivot at c is nearly double the pressure exerted at g.

Figure 32.
Usual position of the fusee and barrel.

        In the other arrangement, shown in Fig. 33, the fulcrum is still at f; the force acts very close to it and the pressure at the pivot c is approximately 1/4 of the pressure exerted at g.

        The difference of the pressure in these two cases is about 8 to 1, and since friction varies with pressure, the advantage which can be obtained by the change is significant; although we should note that the difference between the pressures is largest in these two cases, and when the chain runs on the lowest part of the fusee it will be reduced, but it will still be about 4 to 1.

Figure 33.
Improved position of the fusee and barrel, after Julien Leroy.

        It is surprising that this design (which is undoubtably of great advantage and for which we must thank Julien Leroy) did not find followers in England, the mother country of fusee work, although it was endorsed by Mudge. In contrast, it has been adopted most enthusiastically by German and French chronometer makers.

122. Stopwork is absolutely necessary with fusee work. Its omission is not possible because, on the one hand, the chain could not find space on barrel and fusee if it had a surplus of length; and on the other hand because further winding, for which there would be no limit apart from the excessive strain of the spring, would necessarily unhook the chain from the barrel.         

        The common stopwork for fusee watches is too well-known to need description here. When well executed its action, which happens at right angles, is as strong and safe as we could wish.

123. The stopwork for the arrangement described in Art. 121 must necessarily be different, but at the same time it becomes simpler and better. Instead of the usual stopwork which is pushed, it is pulled at the moment of stopping. It consists of a foot a with the spring arm b lying on a tangent to the circle of the fusee beak. At c it has a wide shoulder, against which the beak acts, with an extension d going in a straight line beyond the chain which, as Fig. 35 shows, is filed thinner and somewhat rounded.

Figure 34.
Common fusee stopwork.

        Now, if the chain is completely wound on the fusee it pushes the spring arm towards the upper plate until the beak meets the shoulder at c and further winding is prevented.

Figure 35.
Fusee stopwork after Julien Leroy.

124. Another kind of stopwork, which is rarely seen, would be suitable for both arrangements of the fusee work and is very solid.                

        It consists of a narrow steel slide a, which fits firmly in a groove in the upper surface the fusee but can slide in this groove. It lies level with the upper fusee-chain groove and may not extend below it. By means of a weak spring b, which is pivoted in the upper surface of the fusee, the slide is pushed back so that its rear end stands out from the upper fusee groove somewhat. When winding, the last turn of the chain takes up its proper position in the highest groove and it presses the slide inward, whereby the other end of the slide c leans against a screw or a strong pin and further winding prevented. Naturally this does not require a fusee beak; instead a round plate is screwed to the fusee which holds the slide and spring in their place.

Figure 36.
Fusee stop work with slide.

125. The stopworks illustrated in Fig. 11-15 are also applicable to fusee watches, but then the stop wheel must be fastened to the plate.        

        Whichever kind of stopwork we use with a fusee watch, we should never forget to protect the more delicate parts of the work from the damage which can result from centrifugal rotation of the free end of the chain when it breaks; either by prudent planting of the frame pillars or by mounting a thin separating wall close to the edge of the barrel.