496 Transactions. 



Art. XLIV. — Apparatus for the Determination of the Magnitude of Small 

 Forces, especially useful in connection with Hydraulic Experiments. 



By Professor E. J. Scott, M.Inst.C.E., M.I.Mecli.E., F.A.I.E.E. 



[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 sur- 

 face. 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 travel- 

 ling 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 string, 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 sus- 

 pended 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 $, 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. 



