186 



DIELECTRIC CONSTANT, ABSORPTION AND SCATTERING 



2. The dielectric constant and conductivity of bulk 

 water for the K band. 



These exiDeriments liave been carried out by Saxton. 

 The method adopted on S and X bands in (1) was to 

 allow superheated steam to isue freely through a 

 resonator into the air. The pressure throughout the 

 apparatus was accordingly the atmospheric pressure. 

 The temperatui-e of the steam was measured before 

 entering and after leaving the resonator. The temper- 

 ature difference between these measurements was no 

 more than 2 C, so it seems clear that no condensation 

 of water droplets could occur within the resonator. 

 A somewhat different technique was employed for 

 X band in that the resonator was replaced by a wave 

 guide. If £ is the dielectric constant, then the best 

 representation of the results is obtained by plotting 

 (£ — 1) against 1/T, for each wavelength where T 

 is in degrees absolute. It was found that the results 

 for different values of T fitted very well to a straight 

 line as they should theoretically, but the value of 

 (£ — 1) for all values of T was found to be systemati- 

 cally aljout 10 per cent less than would have been 

 expected on the basis of previous measurements of the 

 dielectric constant of steam at much longer wave- 



lengths. This discrepancy was found on X and K 

 l)ands. The reason for the discrepancy is not yet under- 

 stood. It is known, however, that the reduced value of 

 (£ — 1) does not arise from strong dispersion occur- 

 ring in the microwave band since there is no evidence 

 of any abnormal absorption. 



The method adopted in (2) was to measure first the 

 attenuation factor of radiation passing through water. 

 This was done by placing a transmitting horn above 

 a large shallow trough and a receiving horn below 

 the trough. The attenuation factor was measured im- 

 mediately by \arying the depth of the water in the 

 trough. Second, the reflection coefficient of electromag- 

 netic waves incident normally on a plane surface of 

 water was measured. These two observations were 

 sufficient to determine the dielectric constant and 

 conductivity of water. The results obtained for a 

 wavelength of 1.58 cm were: 



Dielectric constant about 40; 



Conductivity about 4.10 esu.^^ 

 The same values were obtained for both tap water and 

 distilled water, showing that the presence of salts in 

 the water had little effect on the value of the con- 

 ductivity. 



