m 



(;24 



ii::i'OHT — 1884. 



nnd let us suppose tlmt it nollIuT Iosi'h nnr jrnins hont In- its oiuJs. Lot 11' be the 

 quantity lost by emission from the suii'iice jut unit of tinu*. Let e be tbe eiuissivitv 

 or quftutity of beat lost per unit area of the poolinp^ surface, per unit dillereuce of 

 temperatures betneen tlie cooliu^' surfiice and the surroundinjrs, per unit time ; ami 

 /"" beinpr, as has been said, above the tein])erature of tlie wire, let 6' be the tenipe- 

 rature of the surroundiuj^s. Tiieii : - 



n' = Tr(ll.e.if-0) (:i) 



l^ut when the wire has acquired a pennanent temperature, with the current flowinir 

 throujjh it tliere is as much luT.t beiufj^ lost at the sides an is beinj^ <::enerated bvtbe 

 cm*: nt. In this case II = U' and we obtain the expression for c :— 



e- 



Jn'iPif—ti) 



(4» 



'"ill 



I 



My experiments consist in nieasurinjr the strenf>lh of the current and tlie tompe- 

 rature of tlie wire, the latter bein;; ellected by nieasuriufr the electric resistance nf 

 a known lenpth of tlio wire wliile the ciuTent is llowinfr throiii^h it, and heme 

 inferrinjj the temperature. These lieinj? known, and likewise the temjwraturu of 

 the surroundinfTs, we have all the data for tinding i\ tiie emissivity of the surface in 

 alisolute measure. 



The experiments of Mr. D. .Macfarlane <?ivinfr emissivities in absolute measure 

 are well known, and are of undoubted accuracy. They were communicated to tlie 

 lloyal Society (I'ror. li'ii/. Soc, \f^7-, ]>. !'•">); and the residts are quoted in Prol'. 

 Everett's 'Units and I'liysical ( 'onstants ' (cliap. ix. § 1.".7). These experiments 

 were made with a copper <rlobe about 4 centimetres in diameter, suspended in a 

 cylindrical chamber, with top and bottom, about (50 centimetres in diameter nnd 00 

 centimetres hi'rh. The residts may bo briefly summed up as follows : Mr. .Macfarlane 

 Hnds an emissivity of about ,,',„ of the thermal unit ('. (i. S. per square centimetre 

 per second, per degree of diil'erence of temperatures between cooling body and 

 .surroundings for a polished surface, witli an excess of temperature of a little nnin- 

 than G0°0 ; and, for a blackened surface, the same emissivity withan excess of 0'^ ( ' 

 or under. 



Using round wires of small diameter {OW) millimetre and under), and with tbe 

 surfaces either brightly polished or in the conunon dull condition of a wire 

 IVesh from the maker, 1 have found a much larger emissivity than r„lu,- 1 ''"ve 

 obtained for wires of diU'erent sizes ditlerent values of c varyiu"; from „,}„- down te 

 ,,',y, which was obtained with a wire ot 0*40 mm. diameter, and with an excess ol' 

 temperature of 24° ('. It .seems to be shown by all the experiments I have made 

 that, other things being the same, the smaller the wire tlie greater the emissivity. 



To do away with the part of the emissivity which is due to convection aixl cen- 

 duction by the air, I have commenced experiments on loss of heat by small wires 

 in the nearly perfect vacuum afforded by the modern mercurial air-pump. This 

 part of the subject was experimented on long ago by Dulong and Petit, and within 

 the last few years by Winkelmann and by Kundt and Warburg; lastly, and inucli 

 more perfectly, by Mr. Crookes {I'roc. lioi/. Sor., vo!, xxxi., p. 239), though in mi 

 case, I Ijelieve, were the emissivities in absolute measure determined. The con- 

 clusion come to by all these experimenters is the same, namely, that there i.'^ a 

 decrea.se of emissivity due to lowering of the air pressure, this decrease being very 

 .<mall for a reduction down to one-half, or one-third, of the ordinary atmospiieric 

 pressure, but becoming very great as the vacuum approaches com])letene.'<.«. Tlie 

 very interesting experiments of Mr. Crookes seem to show that, even with tbe 

 higli vacuum wliich he obtained, the effect of the residual gas in carrying off beat 

 from the cooling body was far from being annulled. 



The following table shov.'s the emissivity of a copjier wire with bright .«urfaee 

 lialf a metre long, 0'40 millimetre in diameter, and sealed into a glass tulje about 

 lo centimetre in internal diameter. 



■ill 



