DIRECT MEASUREMENT 



35 



6> 



MODULATING 

 SINE WAVE 



PHASE SENSITIVE 



AMPLIFIER 

 ? 



ERROR SIGNAL 



SAMPLING 

 CAVITY 



SERVO 

 AMPLIFIER 



SERVO 



ERROR SIGNAL 



READOUT 



PHASE SENSITIVE 

 AMPLIFIER 



Figure 2.8. Simplified diagram of the Vetter ref Tactometer. 



such as to permit very nearly a linear function of AA'^ versus probe pene- 

 tration over the range of operation. Once the probe is calibrated with 

 the cavity, the relative calibration is static and practically independent 

 of the contents of the cavity. Although the reference cavity is sealed, 

 there is no necessity for a vacuum. Absolute calibration is accomplished 

 by determining the zero point, or the intercept, where A'' = 0, by evacuat- 

 ing the sampling cavity. 



Since the Vetter refractometer is mechanically tuned by a servosystem 

 having considerable inertia, the high-frequency response is relatively poor. 

 The upper frequency response is 10 c/s. Accuracy and simplicity of 

 operation are its distinct advantages. 



Refractometers have been used to measure both the surface value and 

 the gradient of the refractive index. Although the units so far described 

 were designed originally as ground-based equipment, modifications have 



