temporary working of the line pending the completion of the electrical
arrangements.

Let us now put in a form suitable for calculation the principles with
which Mr. Siemens has illustrated in a graphic form more convenient
for the purposes of explanation, and then show how these principles
have been applied in the present case.

Let L be the couple, measured in foot-pounds, which the dynamo must
exert in order to drive the car, and _w_ the necessary angular
velocity. Taking the tare of the car as 50 cwt., including the weight
of the machinery it carries, and a load of twenty people as 30 cwt.,
we have a gross weight of 4 tons. Assume that the maximum required is
that the car should carry this load at a speed of seven miles an hour,
on an incline of 1 in 40. The resistance due to gravity may be taken
as 56 lb. per ton, and the frictional resistance and that due to other
causes, say, 14 lb. per ton, giving a total resistance of 280 lb., at
a radius of 14 inches. The angular velocity of the axle corresponding
to a speed of seven miles an hour, is 84 revolutions per minute. Hence
L = 327 foot pounds, and _w_ = (2[pi] × 84) / 60.

If the dynamo be wound directly on the axle, it must be designed to
exert the couple, L, corresponding to the maximum load, when revolving
at an angular velocity, w, the difference of potential between the
terminals being the available E.M.F. of the conductor, and the current
the maximum the armature will safely stand. This will be the case in
the Charing-cross Electrical Railway. But when the dynamo is connected
by intermediate gear to the driving wheels only, the product of L and
_w_ remains constant, and the two factors may be varied. In the
present case L is diminished in the ratio of 7 to 1, and _w_