142. There is another mode of examining the action of friction besides
that we have been considering. The apparatus for this purpose is
shown in Fig. 33, in which B C represents the plank of pine we have
already used. It is now mounted so as to be capable of turning about
one end B; the end C is suspended from one hook of the chain from the
“_epicycloidal_” pulley-block E. This block is very convenient for the
purpose. By its means the inclination of the plank can be adjusted
with the greatest nicety, as the raising chain G is held in one hand
and the lowering chain F in the other. Another great convenience of
this block is that the load does not run down when the lifting chain
is left free. The plank is clamped to the hinge about which it turns.
The frames by which both the hinge and the block are supported are
weighted in order to secure steadiness. The inclination of the plane
is easily ascertained by measuring the difference in height of its two
ends above the floor, and then making a drawing on the proper scale.
The starting-screw D, whose use has been already mentioned, is also
fastened to the framework in the position shown in the figure.
143. Suppose the slide A be weighted and placed upon the inclined
plane B C; if the end C be only slightly elevated, the slide remains
at rest; the reason being that the friction between the slide and the
plane neutralizes the force of gravity. But suppose, by means of the
pulley-block, C be gradually raised; an elevation is at last reached at
which the slide starts off, and runs with an accelerating velocity to
the bottom of the plane. The angle of elevation of the plane when this
occurs is called the angle of statical friction.
144. The weights with which the slide was laden in these experiments
were 14 lbs., 56 lbs., and 112 lbs., and the results are given in Table
VII.
We see that a load of 56 lbs. started when the plane reached an
inclination of 20°·1 in the first series, and of 17°·2 in the second,
the mean value 18°·6 being given in the fifth column. These means
for the three different weights agree so closely that we assert the
remarkable law that _the angle of friction does not depend upon the
magnitude of the load_.