Clayton chassis dynos used a big electric generator driven by the wheel rollers and a bunch of resistive strips to load the generator down. By passing the current that the generator was producing through more and more circuits, more power was consumed from the vehicle. When the engine was held to a specific speed at full throttle, a calculation was made of how many watts were being consumed and that figure was converted into a horsepower number.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!