520 



A A TURE 



[March 30, 1899 



necessaty to have the porcelain bulb glazed both inside and out. 

 The glaze becomes sticky, and begins to run at a temperature 

 below uoo C, and the bulb begins to yield slightly and con- 

 tinuously to pressure above this point. (3) With some gases 

 there appear to be slight traces of chemical action or occlusion 

 of the gas by the walls of the bulb at high temperatures. It is 

 for this reason preferable to use the inert gases nitrogen or argon 

 as the thermometric material In any case, the limit of high 

 temperature measurement would be reached when either the gas, 

 or the material of the bulb, began to dissociate or decompose. 

 Deville and Troost, employing CO^ for filling the porcelain 

 bulb, found the temperature of the B. 1'. of zinc nearly 150° 

 higher than with air or hydrogen. This they attributed to a 

 partial dissociation of the CO.2 at the temperature as low as 

 930° C. Some experiments made by the writer appeared, how- 

 ever, to indicate that the effect was due to chemical action 

 between the gas and the porcelain. 



For these and other reasons it appears very doubtful whether 

 any improvement or extension of range can be expected from the 

 use of glazed porcelain. If an attempt is made to employ any 

 of the more refractory kinds of fireclay, there is the difficulty of 

 finding a suitable glaze, and of eliminating leakage and porosity. 

 The writer suggested the use of bulbs of fused silica some years 

 ago (Pioi. Iron and Steel Institute, 1S92), and endeavoured to 

 get such bulbs constructed, but without success. This material 

 possesses many of the requisite qualities, but is for this very 

 reason extremely difticult to work. Metallic bulbs of platinum 

 or platinum-iridium are by far the most perfect in respect of 



PoUCELAlN PyROMETETR 



SecLUFL at Cor D 



< 



IQQ7 



Pkil. Trans. 

 A 3 



Platinurn Wtre^s Inside. 



Fig. 3. 



constancy of volume, regularity of expansion, and facility of 

 accurate construction ; but unfortunately, as Deville and Troost 

 showed, they have such an inveterate tendency for occluding or 

 dissolving gases at high temperatures, that the use of metallic 

 bulbs has been practically discontinued, in spite of their obvious 

 advantages in other respects. 



^■Idvaiilages of " /\estslante"-Metliods. 

 After making many vain experiments, the writer was forced 

 to the conclusion that the ordinary bulb. methods did not promise 

 any satisfactory solution of the problem of extending the range 

 of the gas-thermometer, and that it was necessary to attempt a 

 radically new departure. The optical method, depending on 

 the measurement of the refractivity of a gas at high temperatures, 

 and the acoustical method, depending on the observation of the 

 wave-length of sound, although of great theoretical interest, did 

 not appear to promise sufficient delicacy of measurement or 

 facility of practical application. Kxperiments were therefore 

 made on the methods of eflusion and transpiration, which had 

 been occasionally suggested by previous writers, but have not as 

 yet, so far as the author is aware, been practically investigated as 

 a means of measuring temperature on the absolute .'icale. The 

 method of effusion consists in observing the resistance to the 

 efflux of gas through a small hole or orifice in a thin plate. In 

 the method of transpiration the gas is made to ])ass through a 

 fine tube instead of a small orifice, and the resistance to its 

 passage is observed in a similar manner. These methods may 

 be called " resistance-methods " to distinguish them from the 

 ordinary or " bulb-methods" of pyrometry. They are closely 

 analogous to the now familiar resistance- method of electrical 



NO. 1535, VOL. 59] 



pyrometry, and possess many of the advantages of that method 

 in point of delicacy and facility of application. One very 

 obvious and material advantage, especially for high tempera- 

 ture work, is the smallness and sensitiveness of the instrument as 

 compared with the bulb of an ordinary gas-thermometer. But 

 the most important point of difference, which led the writer to 

 the adoption of these methods, is that the measurements are 

 practically unaffected by occlusion or evolution of gas by the 

 material of the tubes. There is a continuous flow of gas through 

 the apparatus. This flow is very large in proportion to any 

 possible leakage, and it is therefore possible to employ platinum 

 tubes with perfect .safety. 



The Method of Effusion. 



The method of effusion may be very simply illustrated by 

 means of a fine hole in the side of a large and thin platinuitv 

 tube which is heated by an electric current. The current of 

 air is heated in its passage through the tube before it effuses 

 through the orifice. The heated air expands in volume, and 

 the resistance to effusion is increased in proportion to the 

 temperature to which the air is heated. The increa.se of 

 resistance may be shown by means of a gas. current-indicator or 

 " rheoscope," which consists of a delicately suspended vane 

 deflected by a current of gas. A mirror is attached to the 

 vane, and the deflection is measured by the motion of a spo» 

 of light reflected on to a scale, exactly as in the case of the 

 mirror galvanometer, when used for indicating changes of 

 electrical resistance. As a standard of comparison, to show 

 the changes of temperature of the tube, the changes of electrical 

 resistance of the same tube are simultaneously shown by means 

 of a suitable ohmmeter. 



The method of efl^usion is a beautifully simple method, and 

 gives a nearly uniform scale ; but it has two disadvantages, 

 which it shares with the thermo-electric method of measurement. 

 ( I ) It necessarily mea.sures temperature at a point, namely at 

 the point of efi'usion, and cannot be easily arranged to give the 

 mean temperature throughout a space. (2) It is difficult to 

 make the effusion resistance sufficiently large for purposes of 

 accurate measurement. A large resistance means a very fine 

 hole, and it is not easy to satisfy the theoretical conditions of 

 the problem with sufficient accuracy and eliminate the effects of 

 viscosity. 



The Method of Transpiration. 



The method of transpiration is more complicated, and does 

 not give so uniform a scale, or so simple a formula. It has the 

 great advantage, however, that the theoretical conditions of 

 How may be realised with unUmited accuracy, and that the 

 transpiration resistance can be measured with a degree of 

 precision very little, if at all, inferior to the corresponding 

 electrical measurement. 



The complication of the transpiration problem arises from the 

 fact that the flow depends on the increase of the viscosity of the 

 gas, as well as on its expansion. The viscosity of liquids in 

 general decreases very considerably with rise of temperature. 

 That of water, for instance, is six times less at the boiling point 

 than at the freezing point. If the viscosity of gases diminished 

 in a similar manner, it might happen that the transpiration 

 resistance would decrease with rise of temperature. Maxwell 

 was the first to give a theoretical explanation of the behaviour 

 of gases in this respect. On certain simple kinetic assumptions, 

 he showed that the viscosity should increase in direct pro- 

 portion to the absolute temperature. Since the expansioi> 

 follows the .same law, the transpiration resistance on Maxwell's 

 hypothesis should increase in proportion to the square of the 

 temperature. This would give a fairly simple formula, and 

 would make the transpiration thermometer a very sensitive 

 instrument, but the scale would be far from uniform. Maxwell 

 made some experiments on the temperature variation of the 

 viscosity between 0° and 100° C. , which appeared to give 

 support to his mathematical assumptions : but his apparatus did 

 not happen to he of a very suitable type for temperature 

 measurement, and it is clear that he diil not regard this part of 

 his experimental work with great confidence. 



The question of the viscosity of g.ases was next attacked with 

 great vigour in I'lermany by a number of diflerent physicists. 

 They ultimately succeeded in proving that the law was not 

 quite so simple as Maxwell had supposed, and that the rate of 

 increase of viscosity was less than that of volume. A summary 

 of some of the principal results obtained, over the range 0° to 



