52 



DISPLACEMENT INTERFEROMETRY BY 



adjustment for resonance was made at the beginning and not repeated for each 

 individual coil; another is the decrease of sensitiveness in the lapse of time, at- 

 tributed above to temperature, so that initial results are always relatively large. 



35. Further magnification. To improve the apparatus further in the way 

 indicated in 34, it would be necessary to put the vibrating system in vacuo, in 

 order to reduce the damping coefficient. This would have been inconvenient with 

 the present form of apparatus, though steps will be taken toward this end later. 



I therefore proceeded to increase the magnification, using a small microscope 

 objective about one-eighth inch in diameter and weighing but i or 2 grams. 

 Its focal distance was 2 cm. The carriage, etc., were of form already discussed. 

 Some difficulty was experienced in finding a suitable kind of slit. Fine lines 

 ruled on smoked glass, bon ami glass, photographic plate, etc., were much too 

 wide and irregular, subtending about 10 scale-parts in the ocular. Similarly 

 a quartz fiber, seen on a field of transmitted light, was 5 scale-parts broad. 

 The same fiber seen in a relatively dark field and illuminated by side light, 

 gave a bright line, not more than i scale-part broad, and was quite satisfac- 

 tory. The ocular of the telescope was placed at about 40 cm. from the objec- 

 tive. Using one helix of the G coil, and a total resistance of 2,700 ohms, the 

 Siemens dynamometer showed a deflection of 0.3 cm. and therefore a current 

 of i o -3 X 0.4 1 ampere. This current was now weakened by a total resistance 

 of 30,000 ohms when the vibrator still recorded 10 scale-parts. Thus it follows 

 (the inductive resistances being negligible) that a single scale-part corre- 

 sponded to about 5X io~ 6 ampere, which is equivalent to 200 scale-parts per 

 milliampere. The band admitted of an extension to 50 to 1,000 scale-parts 

 before seriously losing demarcation, and there was no greater difficulty in 

 making the adjustment than heretofore. In a non-exhausted environment 

 and close ocular this seems to be the limiting performance. 



TABLE 5. Deflections 5 of dynamometer. Microscope objective; C = o.75Xio- 3 ; 



r = yoo ohms; s = i. 



