BRIDGMAN. — MERCURY UNDER PRESSURE. 409 



zero was usually redetermined directly at every temperature, but in a 

 few cases where it was inconvenient to release the pressure to zero 

 between two sets of readings at different temperatures a calculated 

 correction was applied. This correction was determined from an in- 

 dependent measurement of the temperature coefficient of the zero of 

 one of the four mangauin' coils with which pressure was measured. 

 These four coils were made from contiguous pieces of the same piece of 

 wire. The greatest change of the zero from this effect amounts to 

 400 kgm. between — 20° and 0°. Above zero the correction becomes 

 very much smaller, as this particular kind of manganin shows a maxi- 

 mum resistance at 28°. It will be noticed that it was necessary to 

 keep the bath temperature constant to only 0°.5, in order to remain 

 within the errors of the pressure readings. 



By far the most serious correction is due to the fact that the lower 

 cylinder with the mercury under experiment is at one temperature, 

 while the upper cylinder where the measurements of the change of 

 volume are made is at another temperature, the temperature of the 

 room. It is readily seen that the change of volume during melting, 

 for example, as measured by the displacement of the piston of the 

 upper cylinder, will be in error by the expansion at constant pressure 

 of the liquid kerosene on passing from the temperature of the lower 

 to that of the upper cylinder. This error has the same sign during 

 both melting and freezing. The change of volume so measured will be 

 too large if the lower cylinder is colder than the upper, and too small 

 in the reverse case. The correction evidently becomes rapidly larger 

 with the increase in the difference of temperature, but because thermal 

 dilatation becomes less with increasing pressure, this correction fortu- 

 nately becomes smaller at high pressures, where it is harder to de- 

 termine. The maximum value for the correction in this work was 

 2 per cent over the range -f 20° to — 20°. 



This correction seems to have been entirely overlooked by Tammann. 

 It might very possibly amount at the maximum to over 10 per cent for 

 temperature ranges over 100° such as he employed. The effect of the 

 correction will not be to change at all his co-ordinates of the melting 

 curve, but might possibly modify somewhat the way in which Ar 

 apparently tends toward zero at high temperatures and pressures. 

 This latter point has some theoretical significance in Tammann's theory 

 of solid-liquid. 



The correction had to be determined by direct experiment, as there 

 were no data by which it could be calculated. The discussion, which 

 is somewhat long and involved, will be given in the following separate 

 section. 



