626 lU.l.t. SYS I EM TECHMCAI. JOLKXAI. 



of resistance at 70° (".. ili<niu;h in condiutiN iiy ii ((unpares favorabiv 

 with the accepted nu-ials. Antimony, allhouijli ranked among the 

 metals, is usiialK- to lie found among the excei^lions to any rules laid 

 down for them; the same can be said of hisniitlli. Telltirium is an 

 outstanding instance of an elenuiu with two modifications, and a 

 sample taken al random is likeK lo lie a mixlure of them in un- 

 predictable proportions, which change when it is heated; the char- 

 acteristics are correspondingly crooked, wm] rareh' agree. Iodine 

 has a \er\' higli rt>isiance, 



("om])aring the nu-lals as a grou]) with the non-metals, llie lirst 

 striking rule is that tlieir r(inciurti\ iiii-> ate much higher and rather 

 close together: Irom silver (the most conductixe of all substances at 

 room-temperature), to bismuth, the most resistant of the elements 

 commonK' accepted as metals, the conducting-power descends in the 

 relatixely small ratio of 7.5 to 1. The next and familiar rule is, that 

 increasing temj)eralure and increasing resistance always go together; 

 the characteristic alwaj's slants upward to tiie right, the deri\ati\e 

 and the temjierature-coefficient nf resistance are jxisitive. li is 

 eustomar\' to say that tiie resist, uux' i.s alwa\s .ippro\imati-l\' pro- 

 portional to tile temperature, and tiiat the temper.iture-coefhcient 

 of resistance' alwa\s has approximately the one uni\ersal \aiue, 

 which is the x.ihic of liie temperature-coefficient of volume of an 

 ideal gas at rdnsi.mt j^ressure {or its ienipi-i-.iiuii--((ierii(ient (if pres- 

 sure at const. Hit \nhime). That is id >.i\ . when \\\v temperature of a 

 piece of mel.il i> inm-.ised b\- a gi\cn amouiil, its resist, nice increases 

 approximatei\- in the >ame proportion ,is would tiie pressure of a 

 fixed quantity of an ideal gas. enclosed in .i non-ex|ianding container 

 and raised from the same initial to the s.ime linal ti-mix-ratnre as the 

 met.il. Were the.se statements litir.ilK true, ail the resistance- 

 lemjierature curves for metals would be siniighi lino intersecting the 

 axis of temperatures at absolute zero. But the second statement 

 cannot even be considered a good aiipidxim.iiion, unless one is willing 

 to confer the title "good approximation" on ,i numeric, il \,iluc .()(W(w 

 which is expected to agree with a set of oIimimiI \.iIiks wlii(li r.inges 

 upwards to .00.58 (pota.ssium) and .0003 (iroin. il relr.iin Irom giv- 

 ing a lower limit for the range, for a reason which will presentK be 

 made clear.) .Also the characteristic cur\rs ,irc not rigorousK 

 straight lines, although il is not unre.ison.ibli- to c.ill sonic of them 

 approximaldy straight, when oiu- considirs how wide is ilu- inicr\,il 

 of temperature over which some of them li.i\r bien tr.iced. In 

 some cases a (|uadratic term added to the linear expression, resulting 

 in a formula K= R., + at + hl,- is sufficient to express the data. Usually, 



