208 



this way, we made also a series of experiments with distilled hydrogen 

 from the Lejden laboratory. The quantity of admixtures had been 



estimated at ~ at the most by Prof. Kamerlingh Onnes. We 



5000 



gladly avail ourselves of this opportunity to express our indebtedness 

 to Prof. Kamkrlingh Onnes for his ivindness. In the tilling with this 

 gas the purification apparatus could safely be omitted ; the cylinder 

 was therefore immediately connected with the tube ;;/. The results 

 of the measurements with this agreed within the limits of the errors 

 of observation with the results obtained with the gas purified by 

 us in the way described above. 



^ 3. The measurements. 



How through the different "pressure stages" the gas is conveyed 

 to the measuring tube, has already been described above. Also how 

 the temperature is then kept constant. This is seen by the deviation 

 of the galvanometer, inserted into the Wheatstone bridge. To get 

 a first idea the pressure at which the galvanometer needle deviates 

 is read on the manometer, and a corresponding number of weights 

 is placed on the rotating pressure gauge, after it has been brought 

 in communication with the tubes. If the number of rotating weights 

 is too great, some are taken off till the galvanometer needle has 

 returned to its original zero position. At last a final condition is 

 reached, in which the putting on of 50 grams on the rotating weights 

 makes the needle deviate, while the needle returns to its position 

 of equilibrium when this weight is removed. 



The accuracy with which the pressures are thus measured on the 

 small and the large pressure balance generally amounts to this 25 

 grams up to 900 atmospheres. When the measurement is made with 

 the small pressure balance, which goes up to 250 kg. per cm\ the 

 galvanometer needle may be made to deviate and return by putting 

 on or taking away 10 grams, and even when the contacts are very 

 clean with less. As, however, 25 gr. implies already an accuracy 

 of 1 to 10.000, which is not reached on account of other sources 

 of error, there is no sense in going so far in the determination of 

 the pressure. We only mention the fact as a proof of the very great 

 accuracy of the pressure balance for relative pressure measurements. 



When one pressure measurement has thus been made, the pressure 

 is increased. The galvanometer needle, which now would continue 

 to deviate, must again be brought back to zero, because now another 

 resistance of the volume wire is measured. 



