CONTRIBUTIONS FROxM THE JEFFERSON PHYSICAL LABORATORY, 



HARVARD COLLEGE. 



ON THE PERMEABILITY AND THE RETENTIVENESS OF 

 A MASS OF FINE IRON PARTICLES. 



By B. Osgood Peirce. 



Presented February 14, 1906. Received March 13, 1906. 



In a familiar lecture-room experiment, a mass of iron filings, filling 

 a straight glass tube to a length of thirty or forty times its diameter, is 

 forced to become a rather weak *' bar magnet " by subjecting it to a 

 strong exciting field in a solenoid ; and then, by rearranging the parti- 

 cles, it is made to lose its magnetic moment almost completely, although, 

 if, after the filings have been poured out of the tube, a few of them be 

 examined under a microscope of moderate power, it is usually easy to 

 see that most of the elongated particles retain some magnetism for a 

 good while. A number of persons ^ have studied the magnetic prop- 

 erties of masses of iron filings or of chemically deposited " iron dust," 

 as well as of mixtures of iron particles in various proportions with non- 

 magnetic powders of various kinds. Concise statements of the results 

 of experiments on the subject are to be found in the papers of Maurain 

 and Trenkle. 



I have lately had occasion to measure the permeability and the 

 retentiveness of each of several masses of very fine cast-iron particles, 

 or dust, made by a fine cutting end-mill in a milling-machine, and, as 

 testing the effect upon their magnetic properties of subjecting the 

 particles to a " hardening " process, the results seem to have some 

 slight interest. 



The material to be examined was tamped solidly into a glass tube 

 almost exactly one centimeter in diameter, until a column was formed 

 fifty diameters long. After its ends had been closed by corks, the com- 

 paratively short tube was put into the middle of a long solenoid S, the 



* Tijpler, Pogg. Ann. d. Phys., 1877; v. Waltenhofen, Wied. Ann. d. Phys., 

 1870; Jamin, Comptes rendus, 1875; Bornstein, Pogg. Ann. d. Phys., 1875; 

 Auerbach, Wied. Ann. d. Pliys., 1880 ; Baur, Wied. Ann. d. Phys., 1880; Maurain, 

 ;feclairage, ifelect., 1903; Trenkle, Eriangen. Sitzungsberichte, 1905; Wied. Ann. 

 d. Pliys., 1900. 



