This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
When winding magnet spools on a lathe, the exact amount of wire used can be easily determined by means of the device shown in the illustration. The large reel from which the wire is obtained is conveniently placed on a loose mandrel, or rod, near the lathe, and in line with the spool which is to be wound. A grooved idler wheel, the exact diameter of which is known, is supported between the spool and wire reel so it may freely revolve; the number of its revolutions should be obtained, automatically, by a revolution counter. When using the device, the wire from the reel is placed once around the idler to insure the necessary grip to prevent it from sliding; then it is led to the spool. The exact diameter of the idler being known and the number of revolutions indicated, the true length of the wire wound on the spools can be easily determined by the following formula: Length of wire on spool in feet equals circumference of idler in feet times number of revolutions of idler. - Contributed by C. Swayne, St. Louis, Mo.
Ill: Measuring the Length of Wire on a Spool with the Use of an Idler and Counter