Sec. 3-1] 



MAG X KTH ' T It A AN M'C K US 



213 



^ 



£* 



x 



ft. 



£ 



Fig. (3-1)41. Optical magnetic 

 transducer based upon the 

 orienting effect of magnetic fields 

 upon colloidal particles in a 

 suspension. 



where E t is the amplitude of the incident electric vector, V the Verdet 

 constant, I the length of the ferrite body, and II z the magnetic field 

 strength in the lengthwise direction of the ferrite. The value of /u a , 

 the apparent permeability of the ferrite, is a function of fi, the true 

 permeabilit}'. and the physical size of the specimen; (i a increases 

 linearly with l/d (ratio of length to diam- 

 eter) and approaches jli as l/d approaches 

 infinity. It is desirable to select a ferrite 

 with high //. 



If the ferrite transmits with little loss 

 and the rotation has been very slight, 

 the analyzer will reflect most of the 

 power back to the source through the 

 ferrite. To prevent this reflection and 

 still detect a signal, an absorber must 

 be inserted into the waveguide after 

 the ferrite. which will absorb any com- 

 ponent polarized in the same direction 

 as the incident microwaves. If the fer- 

 rite has a large absorption coefficient, this is not necessary, but in 

 this case the ferrite lowers the intensity, and Eq. (1) must be multi- 

 plied by the e" /a where a is the absorption coefficient. 



The method is applicable to the measurement of magnetic fields 

 in a range from 10~ 5 to 10 oersteds. In the range above 10 oersteds 

 the ferrite tends to become saturated, and the output characteristics 

 become nonlinear. The sensitivity is very high; flux-density varia- 

 tions of the order of 10~ 5 gauss and rotations of 10~ 4 deg have been 

 detected. The field should be uniform throughout the volume of the 

 ferrite. The method is limited by noise introduced by the ferrite, by 

 inhomogeneity of the ferrite, and by temperature variations. 1 



LIGHT ABSORPTION IX COLLOIDAL SUSPENSIONS. Figure (3-1)41 



shows a schematic diagram of an arrangement in which a variation 

 of light absorption in colloidal suspensions takes place under the 

 influence of a magnetic field. 2 A vessel C is placed in the magnetic 

 field between the poles of a magnet. A light beam passes through the 

 two prisms P and through the cell in a direction parallel to the field. 

 The cell is filled with a colloidal suspension of particles that are 

 opaque, anisotropically diamagnetic or paramagnetic, and non- 

 spherical, such as colloidal graphite (diamagnetic) in water. The 

 absorption of light in this suspension depends upon the projected 



1 P. J. Allen, Proc. IRE, 41, 100 (1953), and Rev. Sci. Instr., 25, 394 (1954). 



2 F. D. Stott and A. von Engel, Nature, 161, 728 (1948). 



