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BELL SYSTEM TECHNICAL JOURNAL 



0-10° on a dial. The line dial is connected through reduction gearing to 

 the shaft of the sine condenser. The construction of a phase-shifter which 

 has a sufficiently linear correspondence of electrical phase-shift and me- 

 chanical displacement of a shaft is not practical. Instead a movable index 

 for the fine scale permits correction of the deviation from linearity. The 

 index position is controlled by a corrector which is fastened to the condenser 

 shaft. As the corrector is rigidly associated with the sine condenser posi- 

 tion and not with the scales this permits shifting the linear scales inde- 



lU— (J]>tical cam of phase shifter. 



pendently without affecting the correction. The correction curve (Fig. 10) 

 is printed on a photographic negative which is placed on a transparent Incite 

 drum and projected optically as an index (Fig. 11) adjacent to the line dial 

 of the phase-shifter. The calibration curve is obtained by marking the 

 correction at each calibrating point on a ])iece of cellulose acetate placed 

 on the Incite drum. The correction point is projected on the screen ad- 

 jacent to the fine scale during the calibration and problems arising from 

 divergence or misalignment of the light beam are thus avoided. Since the 

 index is projected upon a surface coplanar with the dial, no parallax exists. 



