THE HYDEAULIC DYNAMOMETER 79, 



disc and to the casing, and since the friction moment of the side glandi 

 also reacts equally on the casing and on the shaft, there is, so lonj 

 as the shaft is rotating uniformly, an exact balance between the driving 

 moment on the shaft and the resisting moment on the casing. Th< 

 latter is provided and measured by the resisting moment of the braki 

 load. 



A slight side clearance between the outer circumference of the whee 

 and of the casing at G G permits of the escape of water into the concentri< 

 chamber H, which is always full when the brake is working. It escape: 

 from this chamber through the automatically regulated valve V^. 



Since the change of momentum varies directly as the quantity o 

 water in motion, and directly as its velocity, the brake resistance may bi 

 varied by varying either the inflow or outflow, a system of levers bein^ 

 arranged as indicated so as to do this if the brake lever rises or falls, am 

 thus to prevent over or under loading. An oil dashpot at P serves t( 

 prevent hunting. 



The object of the air vents may now be noted. The water in motior 

 in each pair of pockets forms a vortex, and in consequence is at a greatei 

 pressure at the outside than at the centre of the pocket. The pressure 

 at the centre may, in fact, in a closed pocket, become less than atmo 

 spheric, while in any case there is a tendency for air to accumulate a: 

 the points of least pressure, so that in the original type the dynamometej 

 gradually emptied itself of water and became air charged. Further, sinc( 

 with a given speed of rotation the pressure from the inside to the outside 

 of the vortices increases at a fixed rate, any change in the interna 

 pressure is accompanied by a corresponding change in the pressure or 

 the pockets of the wheel and casing, and hence affects the resistance. 



By the provision of air vents, however, a constant pressure, equal tc 

 that of the atmosphere, is maintained at the centre of the vortices undei 

 all normal conditions of working ; the brake need not, as in its origina] 

 form, be full of water for satisfactory working ; and for a given speed and 

 water supply the resistance remains uniform. 



This type of dynamometer has many advantages in view of its safety, 

 its accuracy, ease of adjustment, and moderate dimensions. 



The resisting moment varies as A v* r, where A is the combined cross- 

 sectional area of the streams suffering change of direction, v is their 

 mean velocity which is proportional to the angular velocity of the brake 

 and to its radius, and r is the radius of the centre line of the brake 

 pockets. 



It follows that in two simikr dynamometers rotating with the same 



