632 BELL SYSTEM TECHNICAL JOURNAL 



electrical behaxioiir; anil the alloys as a whole, instead of assisting us 

 to understaiul conduction in metals, contribute generously to the 

 already abundant supply of difficulties. It remains to be seen whether 

 the measurements upon single crystals of metals, which are being 

 published at a steadily-increasing rate, are goini; to clarify the sit- 

 uation or increase the perplexity. 



While I have left unmentioned a large number of the phenomena 

 which a theory of conduction must lie required to e.xplain, the few 

 which I have described will give quite an adequate basis for begin- 

 ning a discussion of some of the extant theories. It must be conceded 

 at once that the situation is bad. Perhaps there is some set of assump- 

 tions or of postulates by which the whole chaotic crowd of phenomena 

 can be unified into a harmonious system; but if so, no one has yet 

 formulated it. The theories, such as they are, may be divided into 

 two groups: theories in which the electrons are supposed to move 

 freely within the atoms and be stopped when they reach an inter- 

 space, and theories in which the electrons are assumed to mo\e freely 

 within the interspaces and be stopped when they collide with atoms. 

 Those of the first kind start out with the advantage of being better 

 adapted to the usual effect of pressure on resistance; most metals 

 become more conductive when compressed, as if conduction were 

 assisted by stiueezing the atoms closer together. .Still the oldest, 

 the best-known, and the most highly elaborated <if all the theories 

 belongs to the second kind. This is the one foriiuili\ known as the 

 electron theory of metallic conduction, or more briefly as the electron 

 theory of metals, and quite commonly as the "classical" theory of 

 conduction (it does not take an idea so long to become "classical" 

 in physics as it does in the arts). Founded by Riecke and by Drude 

 in the closing years of the last century, it was developed by Lorentz and 

 has since been worked over by Planck, Wien, Bohr, and other savants 

 of the first eminence. Its popularity is largely due, I suspect, to the 

 fact that it can be formulated with great if specious exactness: that is 

 to say, as soon as a few definite assumptions are made (such as the 

 simple, if unplausible, assumptions that the atoms are big elastic 

 spheres and the electrons little ones), numercial consequences can be 

 calculated with an\- degree f)f jirecision. In this resjiecl most of the 

 competing theories are sadh- defect i\ e. Two or three of the nmnerical 

 deductions made from simple au\iliar\- assumptions have agreed 

 rather well with experimental data; and the\- ha\c contributed to 

 the feeling that there must be some kernel of truth in the mathematics, 

 even if not in the physics of the thing, although it bre.iks down in so 

 many other comparisons with experiment. 



