Cable tensiometer.
https://diqn32j8nouaz.cloudfront.net...r_fullsize.jpg
Previously:
https://www.homemadetools.net/forum/...781#post171033
https://www.homemadetools.net/forum/...485#post177752
https://www.homemadetools.net/forum/...849#post156302
https://www.homemadetools.net/forum/...520#post155036
https://www.homemadetools.net/forum/...651#post118209
