MAGNETOGRAPHY 







Fig. 1. Iron filings oriented along the magnetic 

 lines of force around and between the poles of a 

 small horseshoe magnet placed under surface 

 shown. 



their forces applied to practical problems 

 with tremendous advantage, they have de- 

 fied study by ordinary visual methods. One 

 of the early methods of observing magnetic 

 effects was to sprinkle iron filings above a 

 magnet and allow the filings to orient them- 

 selves along the paths of magnetic force. As 

 illustrated in Figure 1, the filings are aligned 

 with the greatest concentration near the 

 poles. It was with this demonstration and 

 the simple compass test that every student 

 of elementary physics encountered the law 

 — -"like poles oppose and unlike attract." 

 This method of study, although interesting 

 from the gross viewpoint of the external 

 forces, gave no light on the internal forces 

 that must be present and in equilibrium. 



The first relatively successful attempts to- 

 ward "seeing" magnetization, or visually ap- 

 praising the forces that occur in the indi- 

 vidual crystal of a ferromagnetic material, 

 were not made until recent years. In 1931- 

 32, Professor Francis Bitter of the Massa- 

 chusetts Institute of Technology described a 



method by which it was possible to see the 

 separate so-called "magnetic domains" in a 

 specimen. When a piece of ferromagnetic 

 material is demagnetized, a nearby compass 

 needle will be unaffected, and iron filings 

 sprinkled on the sample will not indicate the 

 presence of magnetic fields. When the sam- 

 ple is examined microscopically, however, it 

 is found to consist of small regions, each of 

 which is magnetized to saturation in a cer- 

 tain direction. These regions are called "mag- 

 netic domains". 



In using Bitter's method, which was later 

 used and greatly improved by W. C. Elmore 

 of Swarthmore College, and L. W. McKee- 

 han, formerly of Bell Telephone Labora- 

 tories, a suspension of colloidal magnetite is 

 employed. This is an extremely fine iron ox- 

 ide in a soap solution. When a drop of the 

 colloidal suspension is placed on a freshly 

 polished magnetic surface and covered with 

 a thin glass disc, the magnetic particles are 

 attracted by the strong flux at the domain 

 boundaries. A metallurgical microscope 

 which magnifies about 100-200 diameters is 

 used to see the outlined domains, and the 

 specimen is studied with either "brightfield" 

 or "darkfield" illumination. 



In the brightfield system, the light rays 

 from the source are directed through the 

 microscope objective lens to illuminate the 

 specimen. On return, the reflected light from 

 the specimen passes through the objective 

 lens, thence to the eyepiece to form the final 

 image (Figure 2). In the darkfield system, 



I REFLECTED PAYS 

 I TO EYEPIECE 



^ Min 



LIGHT RAYS 

 FROM SOURCE 



TRANSPARENT 



MIRROR IN VERTICAL 



ILLUMINATOR 



OBJECTIVE 

 COVER GLASS 



. :'0| LIGHT 

 Alb SOURCE 



CONDENSER 



COLLOIDAL MAGNETITE 

 ON SPECIMEN 



Fig. 2. The "brightfield" system of illumina- 

 tion. Reflected light returns through the objective 

 lens, transparent mirror and eyepiece to the ob- 

 server. 



441 



