[Omega](E - CR) (2.)
w = -----------------
f(C)

Let _a_ be the efficiency with which the motor transforms electrical
into mechanical energy, then--

Power required = L w = a E1 C

w
= a C ------- f(C)
[Omega]

Dividing by _w_,

a C f(C)
L = -------- . (3.)
[Omega]

It must be noted that L is here measured in electrical measure, or,
adopting the unit given by Dr. Siemens in the British Association
Address, in joules. One joule equals approximately 0.74 foot pound.
Equation 3 gives at once an analytical proof of the second principle
stated above, that for a given motor the current depends upon the
couple, and upon it alone. Equation 2 shows that with a given load the
speed depends upon E, the electromotive force of the main, and R the
resistance in circuit. It shows also the effect of putting into the
circuit the resistance frames placed beneath the car. If R be
increased, until CR is equal to E, then _w_ vanishes, and the car
remains at rest. If R be still further increased, Ohm's law applies,