1(1 RATIO OF SPECIFIC HEATS. 



The gals aiioineter used in tlieAe expeiiiiifiits imist obviously be provided with 

 a needle which will follow the clmiiges of current in the bridge as quickly as pos- 

 sible. Hence we specially constructed a small galvanometer on the model of Lord 

 Kelvin's instruments. The miiioi' was suspended I'kimi a ipiarlz fibre 5 cm. in length, 

 and canie<l on its rear face a sufficient nuniljer of small lamellar magnets. DeHec- 

 tion.s were read off with ^cale and telescope placed at a distance of about 200 cm. 

 from the galvanometer. 



To guard against the effect of e.xternal magnetic disturbances on the galvan- 

 ometer, we euclo.sed it in a double envelope of soft iron. The walls of this cham- 

 ber were 2-5 cm. in total thickness of solid iron, and were perforated by only 

 a small opening suitable for observing the niirnn\ Two astasizing magnets were 

 specially provided, one of them placed above the galvanometer on a plate with 

 leveling screws, the other 1 ving on the galvanometer bracket. The latter was addition- 

 ally useful as a directing magnet. The period of the needle in tlic tinal e.\i)eri- 

 ments was about 4 seconds, so that a single throw was completed in 2 seconds. 



We estimated the pei-iod of expansion ('' Einspringzeit "), /. e., the time which 

 is to elapse between opening the receiver and closing the galvanometer circuit, 

 ])y the beats of a metronome, and, after some practice, found it fully vouched 

 for to a fraction of a .second. This interval is dependent on the size of the 

 efflux j)ipe. 



^. Reduction of the resistance increment, to^-n>.,, to degrees of teniperature. 



Having found tlic resistance it\, coiresponding to the instant of maximum 

 temj)eiature depre.ssion, we next proceeded with the measurement of 'I\-T.i, the 

 teiujierature equivalent of iv^—io,. For this purjut.se, the Wheatstone bridge w'as first 

 put into balance for the temperatuiv 7, of the bolometer sti'ip, withcmt, however, 

 compressing the gas. Tests were aiipiied to ascertain whether or not the strip 

 resistance was still ?/•, as i-eipiired. Thereafter the resistance at the standaids, Z, 

 was changed to W.,, and a corresponding depression produced, from without, in the 

 temperature of the water bath (and therefore also of the strijt), until the bridge 

 again gave evidence of a balance by reason of alt.sence of current in the galvanome- 

 ter. This condilii>ii was in.iiiitaiiicd I'm' sciiiic tiun'. tlii'oughout the whole of which 

 the needle showed no detiection on closing the bridge circuit. The temperature 

 111" the bath was now read i>IT and considered identical with the temperature 2^ 

 sought. The datum was corroborated by returning again to the initial tempera- 

 ture 7] and noting whether v/', had simultaneously regained its (U-iginnl valne. 



The advantages whi<di we claim foi' tlie present method of reducing ?/',-'/'o to 

 7,- '/.J are not far to seek : In the first place the experiments take but little time, 



