August I, 1878] 



NATURE 



369 



two platinum surfaces, one of which is fixed and the 

 other movable, and in a device for holding the object to 

 be tested, so that the pressure resulting from the expan- 

 sion of the object acts upon the carbon button. 



Two stout posts A, B, project from the rigid base piece, 

 C. A vulcanite disc, D, is secured to the post. A, by the 



platinum-headed screw, E, the head of which rests in the 

 bottom of a shallow circular cavity in the centre of the 

 disc. In this cavity, and in contact with the head of the 

 screw, E, the carbon button, F, is placed. Upon the outer 

 face of the button there is a disc of platinum foil, which 

 is in electrical communication with the battery. A metallic 



cup, G, is placed in contact with the platinum disc to 

 receive one end of the strip of whatever material is em- 

 ployed to operate the instrument. 



The post, B, is about four inches from the post. A, and 

 contains a screw-acted follower, H, that carries a cup, i, 

 between which and the cup, G, is placed a strip of any 

 substance whose expansibility it is desired to exhibit. 



The post. A, is in electrical communication with a galvano- 

 meter, and the galvanometer is connected with the battery. 

 The strip of the substance to be tested is put under a 

 small initial pressure, which deflects the galvanometer 

 needle a few degrees from the neutral point. When the 

 needle comes to rest its position is noted. The slightest 

 subsequent expansion or contraction of the strip will be 



