DIRECT MEASUREMENT 31 



used for comparison, the difference between resonant frequencies becomes 

 a convenient measure of the refractive index variations in the sampling 

 cavity. This method of measurement was used in three different instru- 

 ments: the Grain type, the Birnbaum type, and the Vetter type. The 

 Grain refractometer utiHzes the cavities as frequency determining ele- 

 ments in ultra-stable oscillators. The difference in frequency is then the 

 unit of measure. The Birnbaum refractometer utilizes the cavities as 

 passive resonance frequencies of the cavities. The time difference between 

 resonances is the unit of measure. The Vetter instrument, an improve- 

 ment on Sargent's [16] modification of the Birnbaum's refractometer, 

 utilizes servo techniques to achieve a null system, thus eliminating the 

 necessity for extreme electronic stability. 



The resonant cavities are significant components in any of these instru- 

 ments. Of prime importance is the temperature coefficient of the cavity. 

 Most cavities today are made of invar having a temperature coefficient 

 of approximately one part per million per degree centigrade [17, 18]. 

 This is equivalent to 1 N unit per degree centigrade. Further tempera- 

 ture compensation of the cavity has produced temperature coefficients of 

 0.2 and 0.1 A'^ unit per degree centigrade [19]. Most recently, improved 

 invar cavities, when compensated, yielded temperature coefficients of 

 0.03 N units per degree centigrade [20]. The possibility of improving 

 cavity performance by use of special ceramics [21, 22], with temperature 

 coefficients (without compensation) of 0.1 N units per degree centigrade 

 has been investigated. 



The response time of the refractometer is a function of how much of 

 the end plates of the cavity can be opened to the air without appreciable 

 loss of resonant characteristics. It has been found [23, 24], that as much 

 as 92 percent of the end plate area could be eliminated without serious 

 resonant degradation. Such end plates do not impede the flow of air, 

 and the response time of the instrument is essentially instantaneous. 



In general, the accuracy of the refractometer is at least one order of 

 magnitude smaller than can be achieved by indirect measurement. With 

 proper care, these instruments are capable of discerning changes of the 

 refractive index that are less than a tenth of an A'^ unit. As a relative 

 instrument, i.e., used to measure variations about an undetermined mean, 

 the time constant is such as to easily allow detection of rates of up to 

 100 c/s. 



The development of microwave refractometers led to an immediate in- 

 creased interest in the fine structure of refractive index variations and its 

 application to radio wave propagation. A summary of this development 

 has been given by Herbstreit [25]. Detailed investigations of the inhomo- 

 geneities of refractive index structure using spaced cavities yielded signi- 

 ficant results pertinent to the spectrum of turbulence [26, 27]. Refrac- 

 tometers have been widely used in aircraft [18, 28, 29]. The Grain 



