THE TAIT PRESSURE GAUGE 85 



is applied. By a very simple but ingenious device Tait practically got rid of 

 the disturbing effects of temperature changes in the mercury filling the steel 

 cylinder. He placed within the cylinder a glass tube closed at both ends 

 which all but filled the cylinder. This left the action perfect as a pressure 

 gauge, and rendered negligible its action as a thermometer. Professor Carl 

 Barus in his memoir on the volume Thermodynamics of Liquids 1 found his 

 modified form of the Tait Gauge highly efficient. One great merit was the 

 complete absence of cyclic quality so that the same pressure readings were 

 obtained whether they formed a series of ascending or descending pressures. 



Another example of Tail's ingenuity is his electric contact device for 

 indicating when a definite compression has been produced in a piezometer 

 which is enclosed in an opaque hydraulic press and cannot therefore be seen 

 by the eye. His own description is in these words (second "Challenger" 

 Report, Appendix A). 



" We have, therefore, only to fuse a number of platinum wires, at intervals, into 

 the compression tube, and very carefully calibrate it with a column of mercury which 

 is brought into contact with each of the wires successively. Then if thin wires, each 

 resisting say about one ohm, be interposed between the pairs of successive platinum 

 wires, we have a series whose resistance is diminished by one ohm each time the 

 mercury, forced in by the pump, comes in contact with another of the wires. 

 Connect the mercury with one pole of a cell, the highest of the platinum wires with 

 the other, leading the wires out between two stout leather washers; interpose a 

 galvanometer in the circuit, and the arrangement is complete. The observer himself 

 works the pump, keeping an eye on the pressure gauge, and on the spot of light 

 reflected by the galvanometer. The moment he sees a change of deflection he reads 

 the gauge...." 



Amagat, between whom and Tait much correspondence passed at one 

 time with reference to pressure measurements, adopted this method with great 

 success in his later experiments. Regarding its efficacy he writes 



"Sur la recommandation de 1'eminent physicien, je 1'ai essayd tout de suite et 

 n'en ai plus employe" d'autre, non seulement pour les liquides, mais encore pour le 

 gaz, dans les series allant jusqu'aux plus fortes pressions et pour les temperatures ne 

 depassant pas 50." 



One general conclusion of great interest in these experiments is the 

 representation of the compressibility by an expression of the form Aj(B+fl), 

 where/ is the pressure and A and B depend only on the temperature. In 

 several subsequent papers Tait tested the applicability of this empirical 

 formula to experimental results obtained by other experimenters, notably 

 1 Bulletin of the United States Geological Survey, No. 96 (1892). 



