Wheel of wood, 88"·5 in circumference, on the
same axle as a cast iron barrel of 14"·9
circumference; axle is of wrought iron, 0"·75 in
diameter, mounted in oiled brass bearings; power
is applied to the circumference of the wheel,
load raised by rope round barrel; velocity ratio,
5·94; mechanical efficiency, 5·54; useful effect,
93 per cent.; formula, _P_ = 0·5 + 0·169 _R_.
+-----------+--------+----------+------------+----------------+
| | | | P. | Difference of |
| Number of | R. | Observed | Calculated | the observed |
|Experiment.| Load | power | power | and calculated |
| | in lbs.| in lbs. | in lbs. | values. |
+-----------+--------+----------+------------+----------------+
| 1 | 14 | 2·7 | 2·9 | + 0·2 |
| 2 | 28 | 5·3 | 5·2 | - 0·1 |
| 3 | 42 | 7·7 | 7·6 | - 0·1 |
| 4 | 56 | 10·1 | 10·0 | - 0·1 |
| 5 | 70 | 12·4 | 12·4 | 0·0 |
| 6 | 84 | 14·7 | 14·7 | 0·0 |
| 7 | 98 | 17·1 | 17·1 | 0·0 |
| 8 | 112 | 19·4 | 19·5 | + 0·1 |
+-----------+--------+----------+------------+----------------+
The formula which represents the experiments with the greatest amount
of accuracy is _P_ = 0·5 + 0·169 _R_. This formula is compared with the
experiments, and the column of differences shows that the calculated
and the observed values agree very closely. The constant part 0·5 is
partly due to the constant friction of the heavy barrel and wheel, and
partly, it may be, to small irregularities which have prevented the
centre of gravity of the whole mass from being strictly in the axle.
325. Though this machine is more economical of power than the wheel
and axle of Art 305, yet it is less powerful; in fact, the mechanical
efficiency, 5·54, is only about one-fourth of that of the wheel and
axle. It is therefore necessary to inquire whether we cannot devise
some method by which to secure the advantages of but little friction,
and at the same time have a large mechanical efficiency: this we shall
proceed to investigate.