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DISPLACEMENT INTERFEROMETRY BY 



56. Further observations. Without synchronism in the two vibrating sys- 

 tems (current and telephone), the motion of fringes obtained is practically 

 inappreciable when average currents within the order of milliamperes are 

 treated. This is a curious and at first a disappointing result. As soon, 

 however, as approximate synchronism is established, the sensitiveness of the 

 apparatus increases enormously. It is best for this purpose to vary the period 

 of the motor of the alternate-current generator by the slide rheostat. If the 

 fringes are horizontal and the objective therefore vibrating horizontally across 

 the vertical slit-image, the motion of the fringes is vertical. Hence the hori- 

 zontal band a (fig. 72), in the absence of current, at once takes the form 

 b,c,d,c,b, in succession, with the opposition of rotation in c quite visible. 

 The continuous change of these may be most conveniently accelerated or 

 retarded by controlling the motor of the alternator 

 with the slide rheostat. Since the lines b and d are 

 of different inclination, they will usually show a dif- 

 ference of breadth. Circles appear when the ampli- 

 tude of the objective has sufficiently decreased, and 

 it is advantageous, as a rule, to keep this ampli- 

 tude small, for the phenomenon is then more luminous and brilliant. Very 

 large fringes are not usually desirable, as they are too mobile and may 

 pass out of the field. The smaller fringes are quite satisfactory and more 

 easily obtained. When the tension of the fibers e,e' (fig. 70) is too small 

 the fringes drift, showing that with the varying magnetization there is no 

 persistent position of equilibrium. It is annoying if they leave the field 

 while executing their gyrations, though they may always be restored by 

 moving the micrometer. For mean tensions the higher Lissajous curves 

 2 13, 3 14 may be obtained, both for the alternator moving at smaller and at 

 larger periods than the vibrating mirror. To obtain them the motor running 

 at maximum speed is gradually slowed down by means of the rheostat, when 

 the forms appear in succession, passing through the elliptic series at mean 

 speeds. This in fact is the best tension for practical purposes. The size of 

 the curves and the brilliancy of the whole display is increased by decreasing 

 the damping or lowering the cups p, in figure 70. 



A beautiful phenomenon is observed when the magnets t,t' hold the arma- 

 tures h,h' to the intervening rubber cushions and the fringes are fairly large. 

 The slightest vibration anywhere in the vicinity 

 will then cause the even band a (fig. 72) to change 

 to magnificent large roof-shaped or violin waves. 

 This loose contact device could, no doubt, be made 

 useful for observational purposes. Without the 

 vibration telescope such fringes would not be 

 visible, as they overlap during vibration. 



The Lissajous curves continue to be very marked when additional resist- 

 ances as high as i ,000 ohms are put into the circuit of the alternator. Indeed, 

 they do not vanish appreciably, even for an additional 10,000 ohms, if well 



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o/t 



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