ELECTRICAL WAVE FILTERS 



431 



mechanical axes are all 1.00 centimeter long, whose electrical axes are 

 very thin, being 0.05 centimeter, and whose optical axes vary in 

 dimension from 0.1 centimeter to 1.00 centimeter. In order to eliminate 

 the effect of a series capacitance due to an air gap, the crystals were 

 plated with a very thin coat of platinum. The effect of an added shunt 

 capacitance in parallel with the crystal, due to the electrode capaci- 

 tances, was practically eliminated by running the crystal electrodes in 

 an outer grounded conductor as shown in Fig. 24, which shows the 



100 OHM 

 RESISTANCES 



OSCILLATOR 



OUTER CONDUCTOR 

 V 



i^ 



^ 



INNER 

 CONDUCTOR 



-^±=- CRYSTAL 



INSULATED 

 BEARINGS 



^ 



i 



INSULATOR 



DETECTOR 



OSCILLATOR 



DETECTOR 



Fig. 24 — Measuring circuit used to measure the resonances of a crystal. 



measuring circuit. Contact to the crystal plating is made by means of 

 small electrode points placed at the center of the crystal and kept in 

 place by a small pressure. An increase in pressure over a moderate 

 range was found to have no effect on the frequency of the crystal. 

 The lowering of frequency due to plating was evaluated by depositing 

 several films of known weight on the crystal and plotting its resonant 

 frequency as a function of film weight. The intercept of this curve 

 for a zero plating was taken as the frequency of the unplated crystal. 

 When the frequency of the oscillator was varied, the current in the 

 detector showed frequencies of maximum and minimum current out- 

 put which are respectively the frequencies of resonance and anti- 

 resonance of the crystal. In order to locate accurately the frequencies 

 of anti-resonance, it was found necessary to insert a stage of tuning in 

 the detector, in order to discriminate against the harmonics of the 

 oscillator. For a given crystal the frequency of the oscillator was 

 \aried over a wide range and the resonant and anti-resonant frequen- 

 cies of the crystal were measured. The results of these measurements 

 are shown by Fig. 25. In this curve the bottom part of the line repre- 



