Page "Opisometer" Paragraph 2
from
Wikipedia
A simple opisometer consists of a toothed wheel of known circumference on a handle.
The wheel is placed in contact with the curved line to be measured and run along its length.
By counting the number of teeth passing a mark on the handle while this is done, the length of the line can be ascertained:
Page 1 of 1.
2.202 seconds.