232 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 4 



The spherical shape of the tube permits the measurement of the field 

 strength \F\, regardless of the direction; if the direction of the field 

 is of interest, the spherical tube can be replaced by a longitudinal 



tube. The error in fields that are 

 practically homogeneous over the 

 volume of the sphere (diameter 

 about 1 to 2 cm) is about 2 per 

 cent. The system cannot be used 

 at field strengths below about 20 

 to 100 volts/cm (depending upon 

 the frequency). The indication is 

 instantaneous. 



200 400 



Volts/ cm 



600 



Fig. (4-5)4. Calibration curves of the 

 system shown in Fig. (4-5)3, for differ- 

 ent frequencies of the rf field. 



4-54. Thermal Method in rf 

 Fields 



If a high-frequency field is main- 

 tained in a conductive medium 

 (electrolyte), power will be absorbed in the medium. The power con- 

 centration in watts per cubic centimeter in every volume element of 

 the conductive medium is 



ae 



F* 



V 



8 



77 



where a is the conductivity of the medium, e its dielectric constant, 

 and F the field strength. The power is converted into heat. If heat 

 exchange between different parts of the medium is negligible, the 

 field-strength distribution in the medium can be determined from 

 local temperature measurements. 1 



1 J. Paetzold and P. Betz, Z. ges. exp. Med., 94, 696 (1934); see also K. S. 

 Lion, Arch. Physical Med., 28, 345 (1947). 



