Page "Mainspring" Paragraph 21

A problem throughout the history of spring driven clocks and watches is that the force ( torque ) provided by a spring is not constant, but diminishes as the spring unwinds ( see graph ).

Timepieces, however, have to run at a constant rate to keep accurate time.

Timekeeping mechanisms are never isochronous ; meaning their rate is affected by changes in the drive force.

This was especially true of the primitive verge and foliot type used before the advent of the balance spring in 1657.

So early clocks slowed down as the mainspring ran down.