Art. XLIV.—Apparatus for the Determination of the Magnitude of Small Forces, especially useful in connection with Hydraulic Experiments.
[Read before the Philosophical Institute of Canterbury, 6th Septeember, 1916, received by Editors, 30th December, 1916; issued separately, 10th December, 1917]
Being desirous of conducting some experiments on surface friction, on the resistance of ship-shaped models, and on the impact and reaction of jets, the author devised a piece of apparatus for use in the experimental tilting-tank at Canterbury College, the construction and action of which will be made clear by a description of the surface-friction experiments.
For these, a ballasted plank, with shaped ends, is suspended on a bifilar suspension in a channel, through which a steady stream of water flows The plank has fixed upon it a knife-edge, which engages with a suspended lever provided with a pointer and angular scale. A second knife-edge is clamped to a fine oiled silk line, which is strained over four aluminium pulleys, running on centres, and kept taut by equal weights in scale-pans attached to its ends.
The pulleys are situated two at either end of the tank, the axes of each pair being in a vertical plane, at right angles to the stream and above it. The lower pulley is in each case adjustable vertically. The oiled silk line is thus strained in the medial plane of the stream and slightly above its surface. The down-stream set of pulleys acts simply as a locating and straining mechanism. The up-stream set is mounted on a carriage capable of travelling along the stream, and constitutes both a straining and measuring arrangement.
Measuring is effected by the silk line passing completely round the upper pulley, to the axis of which is fixed a pendulum, which is lifted to a degree proportionate to the torque exercised by the pull on the sting, the angle being read off by means of a pointer and scale. When in use the action of the water on the plank drives it down-stream until the tangential force is balanced by its weight and that of the suspended lever.
The carriage supporting the weighing mechanism is then advanced, thus exercising a pull on the silk line, until the pointer attached to the suspended lever at the plank reaches zero. The angle to which the pendulum on the carriage has been lifted is then noted, and the pull readily obtained from a calibration table.
The fact that the taking of a reading requires a general displacement of the system eliminates the friction of rest, and adds greatly to the accuracy of the observation.
For experiments on impact, the various vane forms are bolted directly to the lever and the weights on the measuring-pendulum are increased.
Since the resistance torque is proportioned to W r sin θ, it is evident that by choosing a suitable weight of pendulum the sensitiveness and capacity of the apparatus can be varied over a wide range. It can be adjusted to measure a variation in pull of 1 gramme, whilst its adaptability is evident from the fact that for a continuation at higher velocities of the surface-friction experiments it is intended to suspend the channel and measure directly the tangential pull of the water on its sides, thus eliminating the wave-making due to the plank-ends.