The chain, 66 feet long made of 100 links, must be held with the same tension when each measurement

http://ncccha.blogspot.com/2008/08/history-of-chain.html

history of the chain

http://www.surveyhistory.org/surveyo r's_chain.htm

or there is this.......

Rugged and at the same time within a tenth....

I happen to have an engineers chain. 100'

I have an "engineer's" chain as well. (I had two but sold one on that auction we had for Derral's coworker. the chain is a funny thing. It is both a physical tool, and a unit of measure. (like 8 oz. cups vs. drinking cups which don't necessarily hold exactly 8 ounces) The unit of measure is 66' based on the "gunter chain" which is a 66' long measuring tool. The Engineer's chain and other length chains are instruments but are not the same length. They can be called chains because they are physical chains.

Now the steel tapes we used when I started surveying we called chains and they were not a real physical chain and they were not graduated to end @ 66' or 100 links of length. How confusing.

Some of the chain operators and their chains were excellent.

In 1900, the county found the distance from this section corner stone to that quarter corner stone to be 20.11 chains. In 1916, a different county surveyor found the distance between the two stones to be 20.13 chains. Last week with modern equipment the distance between the same two stones rounded to 20.12 chains.