RATIO OF SPECIFIC HEATS. 11 



while in the secciul almost nil tlie correctious which are usiiully to be applied 

 in resistance nieasunMnent are liere insignificant, even when teniperatnre increments 

 of .01° C are in (|Ucstion. For neither v/-, nor v)., nor their differences need be 

 known in al)sohit<' vahie. 'I'he position of the sliding l)ridge contact, c, ah)ne 

 requires careful attention. This, together with the resistance standards Z in branch 

 2 of the l)ridge, including the necessary terminal and connecting wires, must be the 

 same and similarl} circumstanced during teniperatui-e measurements, as during the 

 actual measurements relative to clianges of gas pi'essui-c. 



Our resistance standards were made of manganine wire. Thick copper termi- 

 nals and yokes, dipping intt) large mercury troughs, enabled us to connect the 

 individual units of Z at ])leasure. All the connection pieces were stout and of 

 copper wire. Hence changes of temperature in the laboratory were quite without 

 influence on the resistance measurement. 



III. — Systematic Erroks. 



Before proceeding to a report of the experimental data, we will endeavor to 

 form some estimate as to the effect of systematic ei'i-ors on the results. The chief 

 assumption in the present and all preceding and similar methods is fundamental : 

 Even supposing the intrinsic equation for the perfect gas to he fully ap[)licable to 

 the actual phenomena, what assurance have we that the expansion obtained is truly 

 adiabatic? Rigorously considered, none ; for lieat will certainly gain access into 

 the intei'ior of the gas. This heat enters partly by conduction or convection, partly 

 by radiation, and its intiueuce on the results will be such as t() nudce the values 

 found for k smaller than the true values. The influx of heat due to conduction 

 comes partly from the walls of the receiver, partly by direct metallic conduction 

 from the terminals and flaps of the bolometer strip. The former soui'ce of 

 discrepancy occurs nniforndy in all experiments in wdiich measurements are made 

 relative to an expanding gas. It is just in this ivspect that the pi'esent [)iocedure 

 for H measurement has distinct advantages over all earlier methods; foi' these 

 determined 11 indirectly, by the aid of a special pressure measurement; and 

 therefore the total lieat conducted inwards from the walls of the receiver must 

 have entered the results as an error. In our experiments, however, the bolometer 

 strip is suspended in the centre oi a large sphere and that part only of this heat 

 can be effective \\ hich moves as far as the bolometer through the concentric layers 

 of gas. In virtue of the low^ order of heat conduction in gases, the time in which 

 heat can reach the centre is so long an interval, in spite of convection, that appreci- 

 able rise of tempei'ature at the bolometer cannot occur until the expansion is 

 complete and To fully measured. The superiority of the present method of attack 



