The clocks featured in recent posts are built before the general use of electricity. Steam engines would normally be too large and inefficient to power such small devices, plus you would have continually stoke it to keep the clock running over night. So what powers these clocks?
We've discussed examples of the very earliest clocks that run on water power, but the tried and true method of running an accurate timepiece is to harness the power of gravity. This is the original GREEN power. Gravity is constantly renewable and reliable. As long as we have a planet, we have gravity. Weight-driven clocks also have the advantage of being very accurate. The force of gravity is always constant in any particular location, so the clock won't speed up or slow down at any point because of an increase or decrease of power.
Weights get in the way sometimes though. What do you do if you want to make a smaller clock that fits on a shelf? There would be no room to fit the weights. Early American clockmakers solved the problem by making really tall shelf clocks with weights hidden in the sides of the case.
Eventually (in the1400's) the obvious solution became to use the recoiling power of a metal spring. The spring didn't require the space that weights did, but the power of a spring does diminish as the spring winds down. This would cause the clock to slightly slow down - hence the Fusee.
The Fusee is essentially a cone shaped wheel, connected to a powered spring barrel. As the spring applies pressure to the inside of the barrel, it turns, and pulls on the chain connected to the cone. As the chain is pulled from the cone to the barrel less force is required because of the increasing radius of the cone. This compensates for the weakening power of the spring towards the end of its power cycle.
As you can tell, cutting a fusee accurately requires an expert machinist and craftsman. Only the finest, most expert clocks incorporated fusees.
Eventually, finicky and delicate fusee mechanisms became obsolete due to more accurate and more reliable advancements in escapements. Today, very few mechanical clocks have fusees and they have become largely a fascination and curiosity among horologists.
We have several posts that talk about this interesting element of horology. Click here for more posts about the Fusee.