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Mail From: Douglas Hagerman <(email redacted)>

Here's an interesting short article from the February 1998 issue of IEEE
Spectrum magazine:


A herd of horses is plaguing reader Balkan Simsir, who works in Ankara,
Turkey. While he was attending a meeting between two companies, a pair
of engineers began discussion the output power of an electric motor.
Although the engineers could agree on the power in watts, their
calculations differed greatly when it came to expressing the output in
horsepower. Simsir tracked the problem down to a conversion factor
conundrum: one group used 746 W per horsepower, while the other stood by
736 W. The difference of 1.4 percent between the two units became a
source of confusion at the meeting.

Our research turned Simsir's pair of units into a horsepower stampede.
We uncovered no fewer than six types of horsepower, including brake
horsepower, electric horsepower, boiler horsepower, water horsepower,
metric horsepower, and English/Imperial (sometimes referred to as 550
ft.lbf/s) horsepower. Conversion factors for most of these can be found
in the alphabetical list of units in the "Standard for Use of the
International System of Units (SI): The Modern Metric System (IEEE/ASTM
SI 10-1997)."

More sleuthing led to a partial explanation in the Electrical Engineers'
Handbook, Third Edition (John Wiley and Sons, New York, 1936). A
footnote to a power conversion table states: "The horsepower used in
these tables is equal to 550 foot-pounds per second by definition. Other
definitions are one horsepower equals 746 watts (US and Great Britain)
and one horsepower equals 736 watts (continental Europe). Neither of
these latter definitions is equivalent to the first; the 'horsepowers'
defined in these latter definitions are widely used in the rating of
electric machinery."

"I wonder which horsepower is the 'real,' or standard horsepower that is
adopted throughout the world?" asked Simsir. "And how are we supposed to
differentiate between the two units?"

Unfortunately, we have no clear answer. Although the world continues to
move towards the SI system of units, horsepower, in its myriad of forms,
is still commonly used to express the power of electric motors and other
machines. The best suggestion is to note clearly which definition of
horsepower is being used.

Simsir closed his letter with one question about the origin of the
differences between the English and metric (or continental European)
definitions of horsepower: "Out of curiosity, were English horses
stronger than French horses?"