340 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Although different specimens of manganin do not have the same 

 constant, still it may be worth while comparing the results found here 

 with those of other observers so as to give an idea of the magnitude 

 of the variation of the effect. Lafay ^ found 2.16 X 10"' per kgm./cm.^ 

 as the effect on one specimen. Lisell® found for two specimens of an- 

 nealed wire 2.13 X 10~' and 2.08 X 10~* ; for three specimens of hard 

 drawn wire results from 2.279 X lO"' to 2.338 X lO"'. The data of 

 this paper, which are for hard drawn wire, vary from 2.295 X 10~' to 

 2.325 X 10"^ 



Four of these manganin resistance gauges have been in almost con- 

 stant use for over a year. As compared with the mercury gauge they 

 have had the advantage of greater convenience and ease of manipula- 

 tion, even over the pressure range within which the mercury remains 

 fluid. During the work explosions have been of frequent occurrence, 

 the shock of any one of which would have broken the glass capillary 

 containing the mercury. It is not necessary to apply elaborate tem- 

 perature precautions as for the mercury. The only temperature cor- 

 rection necessary to apply is a small one for the shift of the zero. This 

 may be determined accurately enough by hanging a thermometer in 

 the air of the room near the cylinder containing the manganin. At 

 room temperatures of 20° a change of temperature of 1° demands a 

 pressure correction of only 5 kgm. 



As compared with the absolute gauge of the first part of the paper, 

 each form has distinct advantages. Where available, the absolute 

 gauge is more convenient, because it is direct reading and immediate. 

 But the absolute gauge has the disadvantage of leak, so that often the 

 manganin gauge becomes absolutely necessary. Futhermore, it has 

 the disadvantage of being more cumbersome. This means that all 

 parts of the apparatus in connection with the gauge must be corre- 

 spondingly enlarged. This is often a fatal disadvantage, entirely apart 

 from any considerations of expense or convenience, because in order to 

 reach the highest pressures, the steel parts must be hardened, and it 

 is not possible to harden large steel cylinders. The upper limit of 

 pressure attainable will have to be reached with comparatively small 

 apparatus. The reason for not pushing the absolute gauge to its 

 limit was not so much fear of destroying the gauge as the fact that 

 the large 8-inch steel cyHnder containing the gauge was of soft nickel 

 steel, and that the yield point would have been reached at 15,000 kgm. 

 This cylinder had been previously seasoned by applying pressures up 

 to 28,000 kgm., which had the effect of increasing the internal diameter 



"* Lafay, loc. cit. • Lisell, loc. cit. 



