Proving rings are very simple mechanical devices for measuring force. Due to their simplicity, they are often used as calibration standards for other force measurements. Here’s an article from NIST on the history of proving rings (How did the Proving Ring Come About | NIST). Large proving rings are beautiful examples of precision machining. Morehouse is a major manufacturer of proving rings and they still sell them with capacities from 300 lbf to 1,000,000 lbf. Here’s a 300,000 lbf proving ring currently for sale on eBay (Morehouse 300,000 lbf Proving Ring | eBay).