ELECTRICAL RESISTANCE UNDER PRESSURE. 



65 



AR 



Ro 



Ari 



M 



i?3 + rio Ls + lo 



where Lz is the length of the shde wire having the resistance R3, and /o 

 and A^ are the actual readings of the slide wire. This arrangement 

 therefore, gives immediately in terms of slide wire settings the propor- 

 tional change of resistance under pressure. If Rs is kept constant for 

 the runs on the same metal at different temperatures, making the 

 necessary adjustments in passing from one temperature to another by 

 changing only Ri and R^, the readings give directly a comparison of 

 the pressure coefficients at different temperatures without demanding 

 a knowledge of how the resistance itself varies with temperature. 

 This was the procedure followed in this work; the pressure coefficients 



LJ® 



Figure 1. The electrical connections by which resistance is measured. 

 The resistances are so adjusted that the potential drop across R is equal to 

 that across R3 + ri. 



are therefore independent of any error in the temperature coefficient 

 of resistance at atmospheric pressure. 



In actual construction, all parts practical were made of copper. 

 The resistances Ri and Rs were fixed coils dipping in mercury cups in 

 f inch copper bars. The apparatus was so designed that all terminals 

 were within a few inches of each other, in order to avoid thermal 

 effects. The slide wire was of manganin. The slider was also of 

 manganin, and was attached by soldering to a flexible many-stranded 

 manganin wire making connection at the other end to a copper block 

 surrounded by the bars containing the other terminals. By soldering 

 the flexible manganin lead to the slider very close to the point of con- 

 tact with the slide wire, thermal electromotive forces in this part of 

 the circuit were very largely avoided. 



