214 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



area of the graphite particles. A magnetic field has the effect of orient- 

 ing the anisotropically diamagnetic particles; the thermal motion of 

 the particles tends to reestablish random orientation. An exact 

 theory of the process is given by Stott. 1 The optical transmission 

 along the magnetic field increases by about 30 per cent for a field 

 variation from about 100 to 3,000 oersteds. Similar experiments 

 were carried out by Mueller and Shamos 2 with Fe 2 3 in mineral oil 

 and CC1 4 in fields below 10 gauss. At higher field strength saturation 

 sets in. The orientation time for the particles in a magnetic field is 

 relatively short, while the decay time is in the order of 0. 1 to 1 sec for 

 small graphite particles and even longer for Fe 2 3 particles in oil. 



1 F. D. Stott, Proc. Phys. Soc. (London), (B) 62, 418 (1949). 



2 H. Mueller and M. A. Shamos, Phys. Rev., 61, 631 (1942). 



