196 PEOCEEDUM CHB AMERICAS ACADEMY. 



diameter = 0.153 om., the Length varying from 2 to 6 cms., i 

 diameter = 0.115 cm. and a length originally 33. Icma For the shoi 

 specimens he used Gauss's A position, that is, the rod is plan 

 and west and the magnetometer is placed in the prolongation of the 

 rod's axis; for the longer wires Ewing's method was used, in which 

 the soleimid and wire are placed vertically, with an extra solenoid t 

 compensate for the earth's field, and the magnetometer being placed e 

 or west of one end of the wire. 



Du Bois subjected these data to a very extensive discussion. He 

 developed the proposition that, provided the length of the rod a 

 sufficiently great compared with its diameter, then N m a = constant. 

 This constant he finds from Ewing's curves to be equal to 45, provii 

 lit ^ 100. The reason why this formula cannot possibly hold for 

 short roils is that the theory of Du Bois assumes that the aver 

 magnetization intensity / in the whole rod differs but very little from 

 the I within the secondary coil in the middle of the rod ; in other 

 words that the magnetization is practically uniform. Of course this 

 is never realized for finite rods and ordinary fields //', but it seem- at 

 first sight as if the magnetization in a rod of large m should be fairly 

 uniform. If we follow Du Bois's method, which gave him the necessarj 

 data to construct his table of values for X in case of cylinders, we may 

 measure abscissa-differences, which are proportional to X, for the 

 curves for rods of large ill's, and form three or four simultane 

 equations, each of which linearly contains ./•, the abscissa -difference of 

 the normal curve and the /vs. //' curve for the largest m used in the 

 equations. Any two of these equations give x, and we can thus con- 

 struct the normal curve, which gives us immediately all the .V-curve- 

 by plotting abscissa-differences as before. Du Bois, from the meagre 

 data at his command, found values for X for various nt's and has col- 

 lected the results in tabular form (see table, page _<> I) in his book " l>ie 

 Magnetischen Kreise in Theorie und Praxis " ("The Magnetic Circuit 

 in Theory and Practice," translated by Atkinson). He apparently con- 

 siders tin; .Y-curves to straight lines, as far as practical purposes are 

 ooncerned, that is X is not a function of II (or I); at any rate he 

 does not mention any such - variation of X. And as to the question 

 whether or not the X for a given in and / varies with the diamel 

 of the rod, no data were at hand. 



Now there is no reason to believe the A" curves for cylindrical rods 

 of the same diameter to be straight lines ; and since we know that the 

 building up of magnetization, and perhaps even the final result, is very 

 decidedly modified by the bulk of iron magnetized, it is quite likely 

 that thick massive rods of iron really give different values for iVfrom 



