664 ME. C. W. SIEMENS ON UNIFORM ROTATION. 



1. The pedestal containing a battery of two "Maeie Davy's elements" suitably 

 arranged. 



2. The body of the clock, with sides formed of plate glass, containing an electromagnet, 

 by which rotatory motion is imparted to an iron bar or keeper fixed upon the vertical 

 main axis which passes into 



3. the regulating chamber, consisting of a close cylindrical glass vessel with domed 

 glass top containing the rotating cup and a certain quantity of paraffin oil, which fluid 

 is particularly applicable on account of its perfect fluidity and non-affinity for the mate- 

 rials composing the regulator. 



The regulating cup is in this instance formed of vulcanite, and is suspended from the 

 top of the vertical axis by means of a spiral spring, which, being fixed at both ends, not 

 only supports the weight of the cup but acts also as a torsional spring, enabling the cup 

 to descend upon its helical central guide whenever an increase of driving-power calls 

 into existence its equivalent of torsional resistance. 



The rings of stationary and rotating vanes are dispensed with in this instance, because 

 no great variations in the driving-power are contemplated. The electromagnet acts by 

 attraction of the armature during a small portion of its rotation, and one contact only 

 is required, which is so arranged that no destruction of the metallic surfaces can arise 

 through the discharge of extra-current sparks, which latter are received by an elastic 

 point of platinum slightly in advance of the proper contact surface and moved by the 

 same excentric. By this simple arrangement the usual difficulty attending dry contacts 

 is avoided, and a continued action of the instrument ensured. A train of reducing wheels 

 communicates the motion of the cup-spindle to hands upon the face of the clock, which 

 record hours and minutes in the usual manner. 



The diameter of the rotating cup being =0'040 metre, and the height of its edge over 

 the surface of the liquid =0*034 metre, the number n of its rotations per second in 

 accordance with our formula 



^19"6 metre X "038 metre /> n i *• j 



n=- =o*y revolutions per second. 



•040 metre w ^ 



Experiment gave, on the contrary, a speed of 7*5 revolutions per second, or "6 revolu- 

 tion per second more than was indicated by theory, a result which seemed to stamp the 

 action of the instrument with uncertainty, when it was recollected that no allowance had 

 been made for the apertui-e at the bottom of the cup, leaving a portion of the rotating 

 liquid without an external support. 



Correction for lower orifice of Ouj). — If we assume, for instance, that the sides of the 

 cup were cylindrical and merely descended below the surface of the liquid veithout closing 

 in at the bottom, we should find that by rotation of this cylinder, supposing the inner 

 surface to be rough or armed with radial projections, the liquid would rise on the cir- 

 cumference above the external level of surface, but would also form a depression or 

 vortex in the centre of rotation. The surface of the rotating liquid would be that pro- 



