The Hobbs brand is well known worldwide for the development of quality electro-mechanical products. The invention of the first electrically wound automobile clock in 1938 created what is now known as Honeywell Hobbs. John Weston Hobbs was the inventor that cultivated this electrically-wound vehicle clock into a company that has over the years expanded to include hour meters, pressure and vacuum switches, halogen lamps, transmission shifters, and other custom electro-mechanical products.
http://stevenengineering.com/Tech_Su...1HOURMETER.pdf
While searching for the answer, I came across this very nice description of the practical implications of a Hobbs meter:
When oil pressure rises beyond some predetermined minimum value—usually about 10 pounds per square inch absolute—the switch contacts close, the circuit is completed, and at that point, your wallet becomes eligible for a new weight-and-balance.