Commonly referred to as the fencepost error
and occasionally called a "telegraph pole" or "lamp-post" error, it could just as well be called the coil turns error
. If you run an elevated feed-line 100m long with posts 10m apart, how many poles do you need? The intuitive answer 10 is wrong. The line has 10 elevated sections, but 11 poles as illustrated below.
This error often causes confusion when counting the number of turns on a coil. You start at the beginning of the coil, count turns along the edge to the other end, and total is 10. But the total number of turns is not 10; it is only nine, since you have to complete the first turn before you tick off number one, which ends at the second strand of winding that you encounter while counting, not the first. To avoid this error, count the first wire at the very beginning of the winding as the zeroth
turn.NB: Don't be confused by the diagram above that refers to poles in the line or fence in the first example. The diagram would indeed illustrate a 10-turn coil, for which one might erroneously count 11 turns.