Of all the demands placed on instruments, none have been more stringent than those placed upon the mechanical clock. Our expectations are such that if it doesn't keep good time, it is of no use. We think of 99% as being good enough for most things in life. In the measure of the day, which has 86,400 seconds, 99% allows for a variation of 864 seconds, almost 15 minutes. Even 99.99% permitting a variation of 8.64 seconds a day would not be acceptable.

Keeping a pendulum swinging requires a small force to be applied to it, which normally interferes with its operation to a varying extent.

What is so special about the Arnfield clock, invented in 1987, subject of this ModelPlan, is that the pendulum is not required to unlock the escapement, isolating it from the varying pressures of the going train. The pendulum is driven by a gravity arm. There is one gravity impulse arm, and one neutral arm. The escape wheel does not lock alternatively on each side, but only on the neutral arm, which is never touched by the pendulum. It is the gravity impulse arm, after it has finished giving impulse and has separated from the pendulum that impacts the neutral arm and releases the train.

Designed by Dr. Michael Adler the construction of the Meccano model is described in 22 pages of instructions, covering the main plates and assembly, gearing, limit switches, electrical connections, escapement, arms, pendulum, and clock case. A further 33 pages of drawings and diagrams are included, (many drawn with IsoMec), plus 4 pages of coloured photograhs together with a comprehensive Parts List and bibliography.

Also available on CD.   Michael Adler's Clocks on CD.