60 DISPLACEMENT INTERFEROMETRY BY 



Unlike the preceding case, the fringes now obtained were of the elliptic 

 type, so that there is unison between interrupter and vibrator. The ellipses 

 were often magnificent. There was little difficulty in measuring their breadth 

 normal to the direction of fringes when quiet, as this is the fringe displacement. 

 On making or breaking the circuit the ellipses oscillate in the well-known way, 

 and it may take part of a minute or more before they subside into the bands 

 (circuit broken) or into stationary ellipses (circuit made). They remained 

 in the field with resistances up to 10,000 ohms, after which the AA 7 " micrometer 

 had to be adjusted to bring either axial extremity into view. Notwithstanding 

 the feeble current, the telephone was still audible, so that the sensitiveness 

 of the ear has not been exceeded. It is convenient to set the fine slit-image 

 normal to the fringes. The objective vibrates parallel to the fringes, but the 

 major axis of the stationary ellipses is nevertheless usually oblique to these 

 directions, in view of the phase differences of currents. The appearance of the 

 ellipses is often that of disks. When the bands occur, the impression is that 

 of disks seen edge-on. In the absence of resonance the ellipses are imperfect 

 and appear as overlapping half-wave curves. When the degree of resonance 

 between the telephonic vibrator and the vibrating objective is exceptionally 

 perfect, marked ellipses may appear in the absence of current, in spite of the 

 fact that the telescope has an independent mounting. This very annoying 

 phenomenon is apparently hard to eliminate with the observer near at hand. 

 A trace of wax on the vibrator is sometimes a remedy at a sacrifice of sharpness 

 of resonance. Whether the coupling in such a case is merely mechanical or 

 else magnetic remains to be seen. 



In the next experiments another smaller coil with but 10 turns, giving o.ooi 

 volt per turn, was installed with the object of ultimately approaching a condi- 

 tion of silence in an audible telephone. Good vibration ellipses were obtained, 

 both with large and smaller fringes, the latter preferable, because they are 

 usually adequate and more easily controlled. The results were throughout 

 striking and may be exhibited by the aid of figure 62, where ak is the direction 

 of the originally linear slit-image, broadened by the objective vibrating in the 

 direction ab (parallel to the fringes when at rest) to the band abgk, prolonged. 

 The displacement of fringes due to the alternating current being in the direc- 

 tion ak, the ellipse e results. This is often very brilliant, and, except on making 

 and breaking the circuit, quite stationary. If the resistance in circuit is 

 reduced, the ends near a or g of the ellipse may leave the field, unless restored 

 by the micrometer. For large resistances (5,000 to 10,000 ohms) the ellipse 

 shrunk to a line like hf, oblique to ab. The vibration of fringes has not there- 

 fore ceased in the absence of current ; but the two components are now in the 

 same phase and the coupling is apparently mechanical, although the vibrating 

 telescope is on an independent table insulated from the vibrator. As indicated 

 below, it may, however, be magnetic. 



The occurrence of the oblique stationary ellipse indicates a difference of 

 phase between ab, in step with the objective and therefore with the current 

 produced by the impressed voltage in the primary, and ak, in step with the 



