gray: linear expansivity 



249 



whether from thermal expansion or other sources, the direction 

 in which it is stretched should be as nearly as possible perpen- 

 dicular to the displacement. This procedure of arranging the 

 link connecting the accessible with the inaccessible region at 

 right-angles to the displacement is, perhaps, the most important 

 feature of the method; and it is applicable to a greater or less 

 extent in mechanical schemes for transferring motions thru the 

 intervention of rods as well as of wires. 



Two simple arrangements in which the principle has been success- 

 fully applied for determining the linear expansion of bars are repre- 

 sented diagramatically by figs. 1 and 2. In each the expanding bar 



B 



/ 



(I 



Fig. 1. Elongations by method of 

 suspended wires. 



J i 



D 



B 



J 



Fig. 2. Elongations by method of 

 stretched wires. 



is indicated by AB. In the former, wires are freely suspended over the 

 ends of the bar and stretched vertically by the weight of vanes immersed 

 in oil, the viscosity of which is adjusted to damp any swinging of the 

 wires so that their motions will be almost, but not quite, aperiodic. 

 In the latter, suitable for cases in which the bar is immersed in a liquid, 

 wires are stretched upward to another bar CD rigidly connected with 

 the central portion of AB. In both arrangements the motions of the 

 wires are observed through micrometer microscopes focused at con- 

 venient points E and F. A piece of thin white paper, illuminated from 

 behind by a small incandescent electric light form a bright background 

 most agreeable to the eye, on which a wire stands out as a dark bar 

 with smooth, straight edges. Disturbances from changes in level of 



