A clock, both the time train and the strike train, requires a driving force.
In early clocks this power was provided by a weight, on a line wrapped around a barrel. This can be compared to bucket on a wishing well, the weight of the bucket providing a rotational force to the winch. This system has some advantages, it was cheap, easy to produce and it provided a constant force which was important to good time keeping. It is for these reasons that the weight driven clock continues to be produced to the present day.
The weight driven clock did have a couple of disadvantages. Firstly, the clock was not portable and secondly, the clock had to be mounted at a reasonable height to allow for the drop of the weights.
The invention of the clock main spring dramatically changed clock making. It is believed mainsprings appeared in the first spring powered clocks as early as the 15th century in Europe. The first mainsprings were made from a strip or ribbon of steel that was not hardened, and had very limited duration. Clock mainsprings are of a coiled form and are fitted into barrels.
One of the problems related to spring driven clocks is that the mainspring creates more power when it is fully wound then when it is partially wound. The biggest breakthrough in curing this problem was the invention of the fusee.
The fusee is a conical shaped device with a grove cut in it. In the grove a gut line or chain would be wrapped around the fusee and the other end wrapped around the mainspring barrel. At one end of the fusee shaft is the winding square and at the other is a gear that drives the clock train. The idea being that when the mainspring is fully wound the line or chain is acting on the smaller diameter end of the fusee. As the clock trains run, the line unwinds from the fusee and winds back onto the spring barrel. So when the spring is fully wound it is acting on the smallest diameter of the fusee and when the spring is less wound it is acting on the largest diameter section of the fusee. This has the effect of evening out the torque applied to both the clock, critical for timekeeping, and the strike train which helps to produce an even speed for the striking of the bell.
The invention of the fusee revolutionised clock and watch making in Great Britain, and gave rise to the production of many beautiful table or bracket clocks, and pocket watches.