CHAPTER VI. 



THE RAPID TELEPHONIC VIBRATOR ON THE INTERFEROMETER. 



38. Introductory. The preceding apparatus,* with telescopic or micro- 

 scopic enlargement of the telephonic vibrations, behaved on the whole so 

 satisfactorily that it seemed worth while to try a similar design on the inter- 

 ferometer. I was inclined to doubt the feasibility of the plan; but it appeared 

 on trial that the high-tension wires actually keep the auxiliary mirrors of the 

 interferometer practically quiet, so that in the absence of alternating current 

 it is not difficult to find the fringes. Tense wires are out of step with the usual 

 laboratory tremors. The system needs no special damping. 



The displacement of the achromatic fringes due to induced secondary cur- 

 rent is normal to their direction. The objective of the vibration telescope, 

 oscillating in the direction of the fringes, is to be coupled with the impressed 

 alternating voltage. In such a case a full account of changes of phase evoked 

 in any galvanic system under examination may be inferred from the fringe 

 ellipses obtained in the interferometer. The interpretations will therefore be 

 facilitated if a diagram like figure 72 (p. 66), for instance, is kept at hand. 



39. Apparatus. This is in large measure a modification of the apparatus 

 described heretofore, except that special attachments have been added for 

 sharply reaching the resonance tension of the wire. The latter is shown at 

 d, e, e', d f , in figure 58 (front elevation), being the thinnest steel music wire, 



about 0.023 cm - i n diameter. Its ends are wound around the stiff screws b, b' , 

 provided with lock-nuts, and rotating in horizontal short, strong rods a, a', 

 attached to stout standards (not shown) fixed to the bed-plate AB of the 

 interferometer. The wire dd' passes around the grooved pulleys w, w' and 

 above the grooved pulley x, carried in a fork and screw-stem y. The latter 

 may be raised or lowered by the bolt u, which rests upon the massive carriage 

 BB, supported by the slides A A' of the apparatus. Provision must be made 

 (slotted sheath and pins, not shown) to prevent y from turning on its axis. 

 Tension is roughly given to the wire at the screws b,b', and the fine adjustment 

 is thereafter made at the nut u. This worked very satisfactorily. 



* P. N. A. S., iv, 328-333, 1918. Carnegie Inst., Wash. Pub. No. 249, III, chap, v, 1919. 



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