If we could drive the engine so fast as to reduce C' to zero, the whole of the energy of the battery would be available, no heat being produced in the wires, but the horse-power of the engine would be indefinitely small.
It is practically important to consider the rate at which energy may be transformed into useful work, or the horse-power of the agent.
A very convenient rule is to allow one brake horse-power of motor for every to foottons of work done at the hook; this is equivalent to an efficiency of 661%, and is well on the safe side.
The unit of power commonly used by engineers is the horse-power, and this unit corresponds to a rate of working of 550 foot-lb of work per second.
The indicated horse-power developed by a cylinder may always be ascertained from an indicator diagram and observations of the speed.
