662 ' ME. C. W. SIEMENS ON UNIFORM ROTATION. 



tudinally to such an extent as to balance the weight of the cup, which latter may therefore 

 be said to float upon the screw-threads without exercising any pressure upon the same. 

 The upper support of the spindle S is a boss projecting from the bottom of a cylin- 

 drical vessel B of glass sides and glass-domed top, which completely encloses the cup C, 

 while it renders its action visible : this outer vessel is filled with liquid to such a height 

 as to submerge the lower edge of the cup. Rotation of the liquid in the outer vessel 

 is prevented by radial ribs upon its bottom surface ; and upon the external surface of the 

 rotating cup C two concentric projections are provided, one at the upper edge, and the 

 second near the surface of the outer liquid, for the purpose of throwing off some liquid 

 which would otherwise be apt to adhere to the external surface of the cup, in defiance 

 of centrifugal force, and interfere slightly with its proper action. 



Eotation being imparted to the shaft S and the cup C by clockwork or from any other 

 source, the liquid at the bottom of the cup will be acted upon by centrifugal force and 

 rise upon its inner sides, while additional liquid will enter from without and maintain 

 the apex of the liquid curve nearly on a level with the surrounding lake. At the 

 moment when the liquid in rotation touches the upper edge of the cup, the speed should 

 be such as is determined by the formula 



in which h may be taken for the height of the brim of the cup above the lake surface ; 

 but considering that the power necessary to maintain the cup at its velocity, after the 

 liquid has been raised to its upper edge, is exceedingly small, because no fresh material 

 has to be put into motion, it would be practically inipossible to prevent further accele- 

 ration. In order to make sure that the liquid will not fall below the brim of the rotating 

 cup, an excess of driving-power must be applied, and that excess must be disposed of 

 otherwise than in producing further acceleration of the cup. This is accomplished by 

 means of a continual overflow of a thin sheet of liquid, which is projected against the 

 ■ sides of the outer vessel and falls back into the lake at the bottom, whence a similar 

 quantity of liquid penetrates into the cup, to be also raised by its rotation and projected 

 over its edge. 



The power absorbed in raising and projecting the liquid must be strictly proportionate 

 to the quantity of liquid so acted on in a given time, or to the thickness of overflow, 

 provided the condition of uniform velocity be realized ; but the velocity of the cup 

 depends upon the height to which the liquid has to be raised, and must therefore be 

 increased in order to raise the liquid column above the brim of the cup. The necessary 

 increase of velocity to produce such an overflow is not great, considering that the height 

 of the liquid column increases in the square ratio of the velocity, and may be neglected 

 in all cases where only approximate results are required, or where variations in the 

 driving-power are comparatively small ; but no high degree of accuracy could have been 

 claimed for this instrument unless the following compensating action had suggested 

 itself: — 



