Early Dynamometer Measured Horses' Pulling Power

Dynamometers gave a way to standardize measurements at horse pulls

A diagram of the first machine Professor E.V. Collins built on an International Harvester Auto Wagon chassis to test horses for their normal working load.
A diagram of the first machine Professor E.V. Collins built on an International Harvester Auto Wagon chassis to test horses for their normal working load. 
From Bulletin 240 by E.V. Collins and A.B. Caine, published in 1926 by the Agricultural Experiment Station, Iowa State College of Agricultural and Mechanic Arts.