DISC FRICTION 



181 



The casing is prevented from rotating, and its tendency to rotate, 

 which is equal to the resistance to rotation of the disc, is measured by 

 weights applied to a hanger on the one side, and by a light spring 

 balance supporting the other side of the casing. A long, light pointer 

 attached to the casing and work- 

 ing over a graduated scale serves 

 to magnify the readings of this 

 balance. The two parallel sides 

 of the casing are provided with 

 adjustable plates with different 

 surfaces, and are separated by a 

 series of cast-iron rings 13 inches 

 in internal diameter, these form- 

 ing the body and enabling the 

 side clearance to be varied at will 

 from J inch to 2J inches. 



Six plain discs in all were 

 examined, these being either 9 

 inches or 12 inches in external 

 diameter and about 0'2 inch thick. 

 They were formed of polished 

 brass, of rough cast-iron, and of 

 rough cast-iron painted and 

 varnished. The surface of the 

 rough cast-iron discs was left as 

 received from the foundry, except 

 that all outstanding roughness 

 was dressed off. These would be 

 considered excellent castings and 

 had a comparatively smooth skin. 

 After being used they received 



two coats of quick-dry ing varnish- FIG. 93. 



paint, and were then used for the 



third series of experiments. Each disc is carried on a central boss 

 1J inches in diameter and f inch deep. 



The results throughout were very consistent, and there is no reason to 

 suspect an error of more than about 2 per cent, in those of any series. 



Variation of Disc Resistance with Temperature. As it was recognised 

 that it would be impossible to carry out a series of experiments without 

 some variation in the mean temperature of the water, preliminary 



