484 



NEGRETTI AND ZAMBRA, HOLBOEN VIADUCT, E.G., 

 C C A 



FlG. 2956. 



PYEOMETEES. 



2956 Pyrometer, Professor Darnell's, is perhaps the most practically useful; 



fig. 2956 shows its general arrangement. The indications are obtained 

 from the difference in the expansion by heat of an iron or platinum bar 

 and a tube of well-baked black-lead ware, in which the bar is contained. 

 The metal bar, A, is shorter than the tube, and a short plug of earthenware, 

 B, is placed in the mouth of the tube resting upon the iron bar, and so 

 secured by a strap of platinum and a little wedge that it slides with 

 difficulty in the tube. By the expansion of the metal bar the earthenware 

 plug is pushed outwards, and remains in its new position after the 

 contraction of the metal bar on cooling. The expansion of the metal bar 

 thus obtained is measured off by the instrument and index, C, which 

 traverses over a divided circular scale S before the experiment, and after 

 the earthenware plug has been moved outwards by the expansion of the 

 metal bar B. The degrees marked on the scale are in each instrument 

 compared experimentally with those of the mercurial scale, and the ratio 

 marked on the instrument, so that its degrees are convertible into those 

 of Fahrenheit. (Philosophical Transactions, 1830 31,) In use the black- 

 lead cylinder, P, with its metal expansion bar, A, enclosed, is placed into 

 the metal or furnace to be tested, the length of the metal bar being noted 

 on the divided arc, S, previous to the operation, and after its exposure to 

 the heat to be tested. Price 550 



2957 Ferguson's Pyrometer, a lecture table instrument, for demonstrating the 



expansion of metals by heat, the relative amount of expansion of different 

 metals being shown upon the divided Quadrant seen in fig. 2958 



Price, simple form 440 



