630 BEU. SYSTEM TECHKICAL JOURNAL 



the exceptions. This illustrates how the beha\'ior of metals in con- 

 ducting electricity is liable to cut across the classification of tin' 

 Periodic System, which controls nearly all of the properties of elements 

 except those that v'ary uniformly from one element to the next all 

 along the series. 



As for the magnitude of the effect, liie re>istances of niii~t imials 

 are decreased through less than 10'/^ by appKing a jjrc'-snri- of ten 

 thousand atmospheres, some only through one <>r iwo per cenl.; 

 liui the decrease is 40% for sodium, 70'',' for pouissiuni. 70', alx) 

 for ihc "debatable" element tellurium, and '.I7' , fdr lilark phos- 

 phorus; bismuth gains about 2rt% in resistance and aniimon\- about 

 10%. The curves representing resistance as function of pressure 

 are somewhat cur\ed, \m\. not greatly so; howe\er the curvature 

 frequently varies along the curve to such an extent that a two-constant 

 formula is not sufficient to express the data. It is an interesting 

 fact that the percentage by which a given pressure changes the re- 

 sistance of a metal is approximately independent of its temperature, 

 and consequently the percentage by which a given rise in temperature 

 changes the resistance is approximately independent of the pressure; 

 so that the combined effects of a pressure-change A/> and a temperature- 

 change yr on a metal change its resistance from R,, to R,, (l-|-^'A/^) 

 (\+bM'). 



Tension, which is cqui\alent to negative pressure acting along a 

 l>articular direction (there is no way of applying a negative h\dro- 

 static pressure) results in lengthening the metal along one direction, 

 shortening it along all directions perpendicular to that one, and 

 dilating it as a whole. Most of the information about what it does 

 lo electrical resistance is owed to EBridgman. l^sually, but not always, 

 tension increases the resistance to current-flow along the (liitction 

 of the stress. The exceptions are bismuth and strontium. Cinii- 

 |)aring the data about the effects of pressure and of tension, we see that 

 \V\ and Sr are exceptions to the common rules for both, while Li, ("a 

 and .Sb are exceptions to the usual rule for pressure but not to the 

 usual rule for tension. This helps lo show why it is so diflicult to 

 .set up a thoroughly satisfactory theory of conduction in metals. 



By melting a sulistance its density can be altered without altering 

 either its temperature or its pressure; of course, the balance of inter- 

 atomic forces is also altered in some mysterious but very potent way. 

 Melting a solid usually brings about a decrease in density; the solid 

 sinks in the licjuid; but there are exceptions (bismuth, antimony, 

 gallium). The conductivity always changes in the same sense as the 

 density; hence for most metals the solid is more conductive than the 



