169. The wheel is one of the most simple and effective devices for
overcoming friction. A sleigh is an admirable vehicle on a smooth
surface such as ice, but it is totally unadapted for use on common
roads; the reason being that the amount of friction between the sleigh
and the road is so great that to move the sleigh the horse would have
to exert a force which would be very great compared with the load he
was drawing. But a vehicle properly mounted on wheels moves with the
greatest ease along the road, for the circumference of the wheel does
not slide, and consequently there is no friction between the wheel
and the road; the wheel however turns on its axle, therefore there
is sliding, and consequently friction, at the axle, but the axle and
the wheel are properly fitted to each other, and the surfaces are
lubricated with oil, so that the friction is extremely small.
170. With large wheels the amount of friction on the axle is less than
with small wheels; other advantages of large wheels are that they do
not sink much into depressions in the roads, and that they have also an
increased facility in surmounting the innumerable small obstacles from
which even the best road is not free.
171. When it is desired to make a pulley turn with extremely small
friction, its axle, instead of revolving in fixed bearings, is mounted
upon what are called friction wheels. A set of friction wheels is
shown in the apparatus of Fig. 66: as the axle revolves, the friction
between the axles and the wheels causes the latter to turn round with
a comparatively slow motion; thus all the friction is transferred to
the axles of the four friction wheels; these revolve in their bearings
with extreme slowness, and consequently the pulley is but little
affected by friction. The amount of friction in a pulley so mounted may
be understood from the following experiment. A silk cord is placed on
the pulley, and 1 lb. weight is attached to each of its ends: these
of course balance. A number of fine wire hooks, each weighing 0·001
lb., are prepared, and it is found that when a weight of 0·004 lb. is
attached to either side it is sufficient to overcome friction and set
the weights in motion.