A weight-driven clock provides a stable power source over the course of the week that a clock runs because a 1Kg lead weight weighs the same whether ten feet off the ground or five feet. So clocks like Vienna regulators and longcase clocks have a consistent power source throughout the week. A spring, however, stores more power when fully wound than when only half wound; even if you didn't have a clockwork train when you were very young, you'd know that as the spring unwinds, the train gets slower and slower. A spring-driven clock has just the same the problem.
The simplest way to minimise this effect is only to use the top 25% of the spring's power, so if you have a long and powerful spring but keep winding it up when it's power falls to 75% of maximum, it will be a better timekeeper. Most clocks do this; you will probably have noticed that the 8-day clock on your mantelpiece may still be going when you return from a ten day holiday, or even a two week holiday. But it might be five or ten minutes slow, perhaps even more.
Better made clock movements combat this problem by giving the spring less work to do as it unwinds - like taking one of the carriages off the clockwork train mentioned earlier. The clock does this by a clever device called a fusée placed in between the mainspring barrel and the rest of the movement, and this provides a variable gear ratio.
The top picture shows what the bare brass fusée looks like. At its widest end the fusée is cut with teeth to engage with the pinion of the centre wheel arbor (shaft) that turns the hands. As it measures 5 or 6cm in diameter at its widest end, it's pretty easy to spot. Running through the centre you can see the steel winding arbor and the square end is what you put the key on to wind the clock through the hole in the dial each week.
The fusée is helical in shape and cut with grooves - actually there's only one groove but it runs round the cone 14 or 15 times, getting smaller in diameter as it does so - just like there is only one groove on a vinyl record. This groove provides a guide for the 'line' which might be a chain, gut or steel cable that connects the fusée to the large brass barrel that contains the mainspring. Chains were used on the earliest and best made clocks and watches but were expensive to produce as they were handmade. Curiously, the chains were nearly all made by villagers in Christchurch, Dorset and if you wish to know more about the social history behind this, read Allen White's book, "The Chainmakers". Or if you cannot find a copy, try Sue Newman's more recent book, "The Christchurch Fusée Chain Gang".
Lesser clocks were fitted with natural gut line instead of expensive chains, and more recent ones use steel cable, as does the one pictured in the second photograph. Whatever the type, you might be surprised to discover that the line needs to be nearly one and a half metres long.
One end of the line is attached to the mainspring barrel and then wrapped round it, while the other end is attached to the widest part of the fusée. The third photographs shows the cable wrapped round the barrel, which is how you would see it if the clock was fully unwound.
As you then wind the clock, the line is slowly wrapped round the fusée in the continuous groove, pulling against the increasing tension of the mainspring barrel. Once fully wound, all the line is on the fusée, as illustrated in the fourth photograph, taken from the other side of the clock.
The fully wound mainspring has a lot of power so it turns the fusée back again which of course turns the hands. But all that power is directed at the thin end of the fusée so it does not have much leverage. Over the course of the week, the barrel loses some of its power but here's the clever bit: that slight loss in power is compensated for by the increasing leverage of the conical fusée as the line reaches the wider part. So the weakening spring now has less work to do and can deliver a consistent power to the hands throughout the week. Some clocks have two or even three fusées, one for each train so that the strike and chime does not sound slower and laboured towards the end of the week.
So now you can tell if your clock has a fusée movement - just look for the conical fusée between the clock plates near to the mainspring barrel. If you can't see inside the clock case, there are other tell-tale signs to look for while you are winding it. First, it does not get increasingly difficult to turn the key, unlike an ordinary clock, and this is because of the variable gearing provided by the conical fusée. Next, it usually take around thirty half-turns of the key to wind a fusée whereas an ordinary clock will only take six or seven before the mainspring is fully wound again. Also, when a fusée is fully wound, the key will suddenly stop and lock there, not just become really stiff. And fourth, as the final photograph shows, the winding hole in the dial is usually nearer to the centre of the dial on a fusée movement, though you should not rely on this alone.