This article aims to promote a better understanding of brake testing in general and, more specifically, to assist the reader in understanding the basic principles of a brake test dynamometer and how to interpret a Dynamometer Test Report.
In order to discuss how a dynamometer works it is worth looking at its history, as well as understanding the origin and concept of Power, which is the unit that a dynamometer measures.
To the knowledge of the writer the word “Dynamometer” is probably derived from the unit “dyne”. One Dyne is the force required to cause a mass of one gram to accelerate at a rate of one centimetre per second per second in the absence of other force-producing effects.  This unit is rarely used today and is discouraged in favour of the Newton (N) that is part of the SI system.
During the 18th century James Watt, inventor of the steam engine (in about 1775), introduced a unit of power to compare the power of a steam engine with a more familiar source of power. Watt learned “that a strong horse could lift 150 pounds to a height of 220 feet in 1 minute.” This amount of work he called 1 Horsepower.  This unit of power can also be defined as the amount of power required to lift a 550-pound weight one foot high in one second.
During the next 200 years the first dynamometer was designed to measure the brake Horsepower of a motor. This invention was the work of an engineer, Gaspard Clair Francois Marie Riche de Prony (1755-1839). He invented the Prony Brake Dynamometer in 1821 in Paris . Variations of this dynamometer are still in use today. (See the quoted relevant source of this information for a diagram of his dynamometer and accompanying calculations.) – Charles Babbage is also mentioned as the inventor of the Dynamometer.