lg ON THE VISCOSITY OB INTERNAL FRICTION 



The dotted vertical straight lines O, P, Q, R, S, T represent the position 

 of the corresponding lines of equal friction if the disk AB had been accom- 

 panied by an extension of its surface in the direction of B. The total friction 

 on AB, or on any of the curves w, v, w, Ac., is equal to that on a surface 

 extending to the point C, on the supposition that the moving surface has an 

 accompanying surface which completes the infinite plane. 



In the actual case the moving disk is not a mere surface, but a plate 

 of a certain thickness terminated by a slightly rounded edge. Its section may 

 therefore be compared to the curve uu' rather than to the axis AB. 



The total friction on the curve is still equal to that on a straight line 

 extending to C, but the velocity corresponding to the curve is less than that 

 corresponding to the line AB. 



If the thickness of the disk is 2$, and the distance between the fixed 

 disks = 26, so that the distance of the surfaces is 6 j8, the breadth of the strip 

 which must be supposed to be added to the surface at the edge will be 



a = 



(19). 



In calculating the moment of friction on this strip, we must suppose it to 

 be at the same distance from the axis as the actual edge of the disk. Instead 



of A=*-r* in equation (9), we must therefore put A =-r 4 + 27rr'a, aud instead 

 t> ' LI 



of b we must put b /8. 



The actual value of - i* for each surface in inches = 1112'8. 



t 



The value of / in inches and grains was 175337. 



It was determined by comparing the times of oscillation of the axis and 

 disks without the little magnet, with the times of the brass ring (fig. 4) and 

 of the tube and weights (fig. 7). Four different suspension wires were used in 

 these experiments. 



The following Table gives the numbers required for the calculation of each 

 of the five Arrangements of the disks. 



* This result is applicable to the calculation of the electrical capacity of a condenser in the form of a 

 dink between two larger disks at equal distance from it 



