PEIRCE. — DEMAGNETIZING FACTORS OF CYLINDRICAL RODS. 57 



In the case of this iron, the flux density, under an excitation of 2500 

 gausses, would be less than for an infinitely long rod, by about one 

 third of one per cent, for a/b = 30, and, by about one eighth of one 

 per cent, for a/b = 50. 



It has long been known that, although cylindrical rods of soft iron 

 do not become uniformly magnetized when they are exposed longi- 

 tudinally to uniform magnetizing fields, yet they behave in many 

 other respects much like the ellipsoidal pieces which are more easily 

 subjected to analysis. 



A glance at the B vs. H curves obtained thirty years ago by Ewing 

 from an iron wire 0.158 cms. in diameter and originally 47.5 cms. 

 long, shows that under very high excitations the flux through the 

 central cross section of a comparatively short piece of this wire would 

 have been much the same as the corresponding flux through a speci- 

 men several hundred diameters long, but -the demagnetizing factor 

 in the case of such a rod seems to be a function of both the diameter 

 and the length and not a function of the ratio of the two alone, and 

 although at low excitations the numbers given by DuBois and by 

 Shuddemagen are most useful, it is not easy to compute with certainty 

 just how long a specimen must be in order that the flux through its 

 meridian section may be assumed to be unaffected by the nearness of 

 the ends of the rod. It happens that Mr. John Coulson and I, who 

 have been studying the maximum value of / in different kinds of iron, 

 have had occasion to determine the magnitude of the influence of the 

 ends of some rods which we have been using in fields of 2500 gausses 

 and upwards, and we have found that we might have used much 

 shorter test pieces and a much less massive solenoid than we have 



