SOME C0NTF.Mn>R.iNy inr.ixci.s IX I'livsics-r mi 



li(|iii(l, l>til l)i>niiilli. .iiiliini>ii\ . and gallium li.i\x- ^rc.itc-r rcsislaixcs 

 Irozm ill. Ill iiioltt-ii. This is oiir of llu- few nilfs in this fu-ld to whid) 

 nil o\ci'ptiiiiis li.ivr \t't ln'»'n <lisco\t'rf<l. Tlu" olistTNfd \.ilin."s (if tlii' 

 ratio (ri'sislaiiii- of li(|iiid) (n'sislaiu'O of solid), when taliulatt-d and 

 f\aniiiH-d. -.how a ti-ndi'iuA' to ilusltT .ilioul \aUn's which arc ratios 

 of sinipli- inti'i;i'rs. such as 2:1, 1 :;{, 1 :4. It w<nild proliably rtHiiiirc 

 a lartfnl and expert analysis to show whether this tendency is more 

 pronoiinred than a (piile raiuioni distribution might reasonaliK' lie 

 e\|K'cte<l to display. Merciir\' has the highest ratio of all, 4:1. 



Other agencies which arc harder to measure or control may have 

 distressingly great etTccts on the conducli\ity of a metal. The various 

 metallurgical processes, annealing, cold-working and the rest, afTect 

 the resistance; sometimes the sign of the change can be explained by 

 sitying that the process has caused the many small crystals forming 

 the metal to fuse into a few large ones, diminishing the resistance 

 offered by the intercrystalline partitions; sometimes this explanation 

 fails to work. Impurities may have a serious effect; for exampli' 

 Bridgman remarks of bismuth that "a fraction of a per cent, of lead 

 or tin may change the temperature-coefficient from positive to nega- 

 tive and increase the specific resistance severalfold." Often im- 

 purities l>etray themselves by an abnormally low temperature-coeffi- 

 cient of the metal; this means that the absolute rate of increase is 

 unusually small compared to the value of the resistance itself. Tliis 

 is so generally the case that a value of temperature-coefficient which 

 (at 0° (".) is much below, say, .004 is usually taken to mean that the 

 sample of metal under in\estigation is impure; and the "standard" 

 values for individual metals set tlown in tables have often taken 

 sudden jumps upward, when better-purilied samples became avail- 

 .ible for measurements. For this reason I laid more stress, in a 

 preceding paragraph, on the values which far exceed .0030.") rather 

 than the values which fall far below it. .A metal contaminated by 

 .1 small admixture of another metal may be regarded as the limiting 

 case of an alloy. There is an enormous literature of the electrical 

 behaviour of allfiys, and some of the results can be extended to this 

 limiting case. It is found, for example, that if two metals .1 and B 

 form mixed crystals with one another, an alloy formed by mixing a 

 small percentage or a fraction of one per cent of A into B, has a sur- 

 prisingly greater resistance than B; and vice versa. The temperature- 

 coefficient of the alloy is on the other hand much smaller than that 

 of the metal, and may even be negative. Thus, although an alloy 

 of this type may seem to be as thoroughgoing a metal as either of the 

 |)ure elements of which it is ma«le, it has a thoroughly anoin.ilous 



