158. We shall next inquire how it is that we are enabled to obviate
friction by means of a pulley. It is evident we must provide an
arrangement in which the rope shall not be required to slide upon an
iron surface. This end is attained by the pulley, of which we may take
I, Fig. 34, as an example. This represents a cast iron wheel 14" in
diameter, with a V-shaped groove in its circumference to receive the
rope: this wheel turns on a ⅝ inch wrought iron axle, which is well
oiled. The rope used is about 0"·25 in diameter.
159. From the hooks E, F at each end of the rope a 14 lb. weight is
suspended. These equal weights balance each other. According to our
former experiment with the studs, it would be necessary for me to
treble the weight on one of these hooks in order to raise the other,
but now I find that an additional 0·5 lb. placed on either hook causes
it to descend and make the other ascend. This is a great improvement;
0·5 lb. now accomplishes what 33 lbs. was before required for. We
have avoided a great deal of friction, but we have not got rid of it
altogether, for 0·25 lb. is incompetent, when added to either weight,
to make that weight descend.
160. To what is the improvement due? When the weight descends the rope
does not slide upon the wheel, but it causes the wheel to revolve with
it, consequently there is little or no friction at the circumference of
the pulley; the friction is transferred to the axle. We still have some
resistance to overcome, but for smooth oiled iron axles the friction is
very small, hence the advantage of the pulley.
There is in every pulley a small loss of power from the force expended
in bending the rope; this need not concern us at present, for with the
pliable plaited rope that we have employed the effect is inappreciable,
but with large strong ropes the loss becomes of importance. The amount
of loss by using different kinds of ropes has been determined by
careful experiments.