Power - Most smaller clocks work from springs however in long case clocks and their wall-mounted equivalents the power is released from weights. There are some interesting variations - "gravity" mantel clocks make a feature of the fact that the weight is the body of the clock, usually falling very slowly down a fixed pole. The most important mechanical part is the escapement - the component that ensures smooth release of the power. It is a series of gears, which transforms the power in the mainspring or weights into the precise release needed to keep accurate time. The final control is by way of a pendulum or its more compact equivalent, the balance wheel. There are typically around 60 components in the movement of a striking clock.
What Makes Some More Valuable than Others?
Not accuracy for certain - electricity wins hands down every time. Early ingenuity and the subsequent investment of time, money and craftsmanship make the earliest of their kind the most sought after. Quality is vital - in both the mechanism and its housing. Most clocks are the product of several craftsmen; the horologist, the precision metalworker, the cabinet maker, founder or stonemason (cases), the jeweller, gilder, glazier and potters (for the dials and other details. The qualities of the dial (clarity and condition), case (marble rather than slate, solid wood rather than veneer) and any decorative features such as statues or finials (bronze rather than spelter) combine to determine the desirability and therefore the value of the whole.
Art Deco, Art Nouveau and honest country pieces are just as much the `in` thing in the clock world as they are in mainstream antiques. Originality of the components - an important consideration with longcase clocks. Does the dial match the movement, and then does the clock match the case?
Check that there is tension in each of the springs and that the clock makes some sort of noise at the half or full hour (it needn`t be a chime, nor does the number of thuds need to match the time displayed). Check for damage to any ceramics or enamel - particularly the dial. Although those checks are no guarantee, they go a long way to avoid clocks in need of costly repairs.