( 303 ) 



^ 5. The moiuiünri inith fJu' (/iferential (j'i/rdnnmcfi'r. The niOiiiiling 

 as it liiialh" was made is fcpreseulfd in l'l. 1, lii;. o. The (•(tiumulator ('j 

 serves for the eleniciil. Tiie coiimiiilalor ( '., ('iial)les the observer 

 to compare eilher liie philiiiiiiii wire wilh Ihe resislaiice /t\ 

 (complete Unes), or 11 ie i^old wiiv willi Ihe philiiiuiii wire ((h)tted 

 lines). It is convenient that by a single commnlalion we should he able (o 

 interchange these two mountings, l)ecause lor the comparison of 

 gold with platinum u ires it will be alwaj'S desirable to know 

 the approximate value of the platinum resistance for (he determination 

 of temperature (cf. § (S). ( '., is a commutator to interchange the 

 two condncts of the diiferential galvanometer. This is necessary for 

 the determination of the ratio between the currents in the two 

 not perfectly equivalent conducts when the deflection is zero and 

 moreover it seemed desirable to me to test conliuually whether this 

 ratio remained nnchanged. 



The galvanometer was tirst a thin wire Thomson with two pairs 

 of coils each with a single wire. To attain the required synnnetry, 

 the four coils were replaced by new ones, in each of which two 

 wires were wound together. This was done in the worksho[)s of 

 the laboratory. The sensibility was a deflection of 1 mm. on the scale 

 with a difference in intensity of current of JO"'^ Ampère, period of 

 oscillation 20". Then the galvanometer was aperiodic. T.ike the 

 galvanometer of the Whkatstonk's bridge in the previous [)ai)er, 

 this was protected against distnrbances arising from terrestrial mag- 

 netism by a soft iron ring. 



The resistance boxes B\, R, and R,,, are all of manganin wire, 

 having therefore an almost negligible tenq)crature coeflicient ; R\ 

 is the carefully investigated resistance box of Hartmanx and Braun, 

 also used in the measurements with the Whkatstonk's bridge; R, 

 is the box wonnd in onr own laboratory, wiiich i)i-oduced two 

 branches in tlie Wheatstone's bridge (cf. previous i)ai)er), of 

 which I also determined the absolute values, /t, is a resistance 

 box of Siemens and Halske tested by the Reichsanstalt. I ha\e 

 determined a few times the ratio between the units R\ and R^ and 

 found 1.00255 (11 determinations, greatest... 264, smallest... 242). 



§ 6. The measurements at low temperatures. x\fter the zero of the 

 wire that was to be com])ared, had been determined a few times, and 

 sufticiently harmonizing results were obtained, the wire was placed 

 into the cryostat inside the platinum thermometer at the place occupied 

 by the hydrogen thermometer when the former was calibrated (cf. 

 Comm. N". 77, § 4 and N". «3, PI. II). 



When enough liquelied gas was poured oif, the ratio between the 



