«ot; 



Undoubtedly tliere are also points of difference, which must be 

 chiefly owing to this that in our case the so-called bearing-pressure 

 is wanting on account of the vertical position of the piston. Appli- 

 cation of the theory taught that if the peripherical speed is sulHicient, 

 a liquid layer will he formed everywhere l)etween piston and 

 hole-wall. The number of revolutions at which this takes place, will 

 be called the critical value of the revolutions w,. It is dependent on 

 the viscosity of the liquid chosen. In the absence of any metal 

 contact also the axial friction would be a liquid fiiction above this 

 value of revolutions. 



In order to test ihe validity of this theory the pressure balance 

 of the Van deh Waals fund which was at our disposal, was modi- 

 fied in such a way that it had a driving apparatus thai could be 

 regulated mechanically. 



This alteration was made by the instiument-maker of the labora- 

 tory, Mr. .1. Wassicnaak. 



Characteristic of a liquid friction is its proportionality with the 

 xelocity. When a definite initial value of revolutions 12 is given 

 to the piston, after which the motor is cut o\it, the motion will be 

 retarded, and the angle x passed over in the time t, will get a 

 value of 



„ = -(!-«-- 



in which ^ is a constant. As soon, as the value of revolutions 

 descends below the critical value however, there is metal contact, 

 and tiie image of the motion changes. 



In this way the course is examined all over the measuiing scope 

 of the pressure balance, and agreement was found between expe- 

 riment and theory. As was to be ex|)ected, the critical value of the 

 revolutions then appeared to be dependent on the temperature, as 

 this influences Ihe viscosity, but independent of the load. 



An electi'ical determination shows the validity of the suppositions 

 still more clearly. For, when the electrical resistance between axle 

 and wall was measured, it appeared to be about 700 Ohms above 

 a definite number of revolutions, being reduced pretty suddenly to 

 0.2 Ohm on diminution of the velocity. In these values the resistance 

 of the conducting wires is included. 



Conclusion. For a favourable use of the pressure balance experi- 

 ments should always be made above the critical value of 

 revolutions. This value can be determined experimentally for every 

 liquid and temperature. 



