488 



Professor F. T. Trouton 



[March 11, 



this case the same kind of motion as contemplated in the definition 

 above referred to. The subjoined table exhibits the determinations 

 made in this way for five of the substances in the series. 



Torsion MetJwd. 



The viscosity in the case of the more viscous materials of the 

 series was in the first instance determined by a method in which a 

 column or rod was twisted round and round, one end being held. 

 From the amount of the twist required to force it to turn at a given 

 rate, the viscosity can be calculated. The table shows the value 

 found in this way for several substances. 



The apparatus employed in these determinations consisted in a 

 horizontal shaft turning on anti-friction wheels, round which a cord 



Torsional Force removed. 



lOo eoo 300 



T I ML tin Seconds' 



500 600 



Fig. 4. 



carrying a weight is wound. The weight unwinding the cord turns 

 the shaft. A rod of the material to be tested is fixed in line between 

 the end of the shaft and a stationary clamp. The rate at which the 

 shaft turns is measured from a divided dial face. A knowledge of 

 this and of the weight enables the viscosity to be calculated. 



Fig. 3 will help us to understand that there is a direct con- 

 nection between the simple alteration in shape dealt with in the 

 definition of viscosity and the effect produced on the substance of the 

 rod when twisting. The small square is seen in the second figure to 

 have become bent in consequence of the twist imparted to the rod. 



The results obtained with this apparatus are shown in Fig. 4. 

 The curve shows the angle through which the rod had twisted at any 

 moment after the application of the force. 



