the rate of 50 feet per minute, against gravitation, it would expend
33,000 foot pounds per minute in moving itself, and although this
machine may give 2 horse power, with an efficiency of 90 per cent.
it would, in the case of a boat or a tram-car, be termed a wasteful
machine. Here we have an all-important factor which can be neglected,
to a certain extent, in the dynamo as a generator, although from an
economical point of view excessive weight in the dynamo must also be
carefully avoided.

The proper test for an electro-motor, therefore, is not merely its
efficiency, or the quotient of the mechanical power given out, divided
by the electrical energy put in, but also the number of feet it could
raise its own weight in a given space of time, with a given current, or,
in other words, the number of foot pounds of work each pound weight of
the motor would give out.

The Siemens D2 machine, as used in the launch shown in the diagram on
the wall, is one of the lightest and best motors, it gives 7 horse power
on the shaft, with an expenditure of 9 electrical horsepower, and it
weighs 658 lb.; its efficiency, therefore, 7/5 or nearly 78 per cent.;
but its "coefficient" as an engine of locomotion is 351--that is to say,
each pound weight of the motor will yield 351 foot pounds on the shaft.
We could get even more than 7 horse power out of this machine, by either
running it at an excessive speed, or by using excessive currents; in
both cases, however, we should shorten the life of the apparatus.

An electro-motor consists, generally, of two or more electro-magnets so
arranged that they continually attract each other, and thereby convey
power. As already stated, there are numerous factors, all bearing a
certain relationship to each other, and particular rules which hold