3 66 



NA TURE 



\_Atigust 14, \\ 



shark is stated to be of mild disposition and quite 

 harmless. Indeed, the minute size of its teeth has led 

 to the belief in the Seychelles that it is a herbivorous 

 fish, which, however, is not probable. 



Albert Gunther 

 Natural History Museum, July 30 



PYROMETERS 



"PHE accurate measurement of very high temperatures 



•*■ is a matter of great importance, especially with re- 

 gard to metallurgical operations ; but it is also one of 

 great difficulty. Until recent years the only methods 

 suggested were to measure the expansion of a given fluid 

 or gas, as in the air pyrometer ; or to measure the con- 

 traction of a cone of hard, burnt clay, as in the Wedg- 

 wood pyrometer. Neither of these systems were at all 

 reliable or satisfactory. Lately, however, other principles 

 have been introduced with considerable success, and the 

 matter is of so much interest not only to the practical 

 manufacturer but also to the physicist, that a sketch of 

 the chief systems now in use will probably be acceptable. 

 He will thus be enabled to select the instrument best 

 suited for the particular purpose he may have in view. 



The first real improvement in this direction, as in so 

 many others, is due to the genius of Sir William Siemens. 

 His first attempt was a calorimetric pyrometer, in which 

 a mass of copper at the temperature required to be 

 known is thrown into the water of a calorimeter, and the 

 heat it has absorbed thus determined. This method, 

 however, is not very reliable, and was superseded by his 

 well-known electric pyrometer. This rests on the prin- 

 ciple that the electric resistance of metal conductors 

 increases with the temperature. In the ca e of platinum, 

 the metal chosen for the purpose, this increase up to 

 1500° C. is very nearly in the exact proportion of the rise 

 of temperature. The principle is applied in the follow- 

 ing manner : — A cylinder of fireclay slides in a metal 

 tube, and has two platinum wires tooth of an inch in 

 diameter wound round it in separate grooves. Their ends 

 are connected at thetoptotwo conductors, which pass down 

 inside the tube and end in a fireclay plug at the bottom. 

 The other ends of the wires are connected with a small 

 platinum coil, which is kept at a constant resistance. A 

 third conductor starting from the top of the tube passes 

 down through it and comes out at the face of the metal 

 plug. The tube is inserted in the medium whose temper- 

 ature is to be found, and the electric resistance of the 

 coil is measured by a differential voltameter. From this 

 it is easy to deduce the temperature to which the platinum 

 has been raised. This pyrometer is probably the most 

 widely used at the present time. 



Tremeschini's pyrometer isbased on a different principle, 

 viz. on the expansion of a thin plate of platinum, which 

 is heated by a mass of metal previously raised to the 

 temperature of the medium. The exact arrangements 

 are difficult to describe without the aid of drawings, but 

 the result is to measure the difference of temperature 

 between the medium to be tested and the atmosphere at 

 the position of the instrument. The whole apparatus is 

 simple, compact, and easy to manage, and its indications 

 appear to be correct at least up to 8oo° C. 



The Trampler pyrometer is based upon the difference in 

 in the coefficients of dilatationjor iron and graphite, that 

 of the latter being about two-thirds that of the former. 

 There is an iron tube containing a stick of hard graphite. 

 This is placed in the medium to be examined, and both 

 lengthen under the heat, but the iron the most of the two. 

 At the top of the stick of graphite is a metal cap carry- 

 ing a knife-edge, on which rests a bent lever pressed 

 down upon it by a light spring. A fine chain attached to 

 the long arm of this lever is wound upon a small pulley ; J 

 a larger pulley on the same axis has wound upon it a 



second chain, which actuates a third pulley on the axis 

 of the indicating needle. In this way the relative dilata- 

 tion of the graphite is sufficiently magnified to be easily 

 visible. 



A somewhat similar instrument is the Gauntlett pyro- 

 meter, which is largely used in the north of England. 

 Here the instrument is partly of iron, partly of fireclay, 

 and the difference in the expansion of the two materials 

 is caused to act by a system of springs upon a needle 

 revolving upon a dial. 



The Ducomet pyrometer is on a very different principle, 

 and only applicable to rough determinations. It con- 

 sists of a series of rings made of alloys which have slightly 

 different melting-points. These are strung upon a rod, 

 which is pushed into the medium to be measured, and 

 are pressed together by a spiral spring. As soon as any 

 one of the rings begins to soften under the heat, it is 

 squeezed together by the pressure, and, as it melts, it is 

 completely squeezed out and disappears. The rod is then 

 made to rise by the thickness of the melted ring, and a 

 simple apparatus shows at any moment the number of 

 rings which have melted, and therefore the temperature 

 which has been attained. This instrument cannot be used 

 to follow variations of temperature, but indicates clearly 

 the moment when a particular temperature is attained. 

 It is of course entirely dependent on the accuracy with 

 which the melting-points of the various alloys have been 

 fixed. 



Yet another principle is involved in the instrument 

 called the Thalpotasimeter, which maybe used either with 

 ether, water, or mercury. It is based on the principle 

 that the pressure of any saturated vapour corresponds to its 

 temperature. The instrument consists of a tube of metal 

 partly filled with liquid, which is exposed to the medium 

 which is to be measured. A metallic pressure gauge is 

 connected with the tube, and indicates the pressure exist- 

 ing within it at any moment. By graduating the face of 

 the gauge when the instrument is at known temperatures, 

 the temperature can be read off directly from the position 

 of the needle. From 100" to 220° F. ether is the liquid 

 used, from thence to 6So° it is water, and above the latter 

 temperature mercury is employed. 



Another class of pyrometers having great promise in 

 the future is based on what may be called the " water- 

 current " principle. Here the temperature is determined 

 by noting the amount of heat communicated to a known 

 current of water circulating in the medium to be observed. 

 The idea, which was due to M. de Saintignon, has been 

 carried out in its most improved form by M. Boulier. 

 Here the pyrometer itself consists of a set of tubes one 

 inside the other, and all inclosed for safety in a large tube 

 of fireclay. The central tube or pipe brings in the water 

 from a tank above, where it is maintained at a constant 

 level. The water descends to the bottom of the instru- 

 ment and opens into the end of another small tube called 

 the explorer (explorateur). This tube projects from the 

 fireclay casing into the medium to be examined, and can 

 be pushed in or out as required. After circulating through 

 this tube the water rises again in the annular space 

 between the central pipe and the second pipe. The 

 similar space between the second pipe and the third pipe 

 is always filled by another and much larger current of 

 water which keeps the interior cool. The result is that 

 no loss of heat is possible in the instrument, and the water 

 in the central tube merely takes up just so much heat as 

 is conducted into it through the metal of the explorer. 

 This heat it brings back through a short indiarubber pipe 

 to a casing containing a thermometer. This thermometer 

 is immersed in the returning current of water and 

 records its temperature. It is graduated by immersing 

 the instrument in known and constant temperatures, 

 and thus the graduations on the thermometer give 

 at once the temperature, not of the current of water, 

 but of the medium from which it has received its heat. 



