94 



DISPLACEMENT INTERFEROMETRY BY 



The telephonic system was now put in place of the galvanometer of a 

 Wheatstone bridge, the small induction-coil being used as a source of cur- 

 rent. It was then found that the adjusted resistances of the bridge could be 

 changed by no more than a few tenths of i per cent before the even band of 

 fringes changed appreciably to the wave-form aa (fig. 68). But as a dyna- 

 mometer the instrument was still much inferior to the audible telephone. Cur- 

 rents of an order less than an average icT 4 ampere would be difficult to detect. 



55. Bifilar systems. To utilize such a system as figure 69 to full advantage 

 it would be necessary to attune the springs ab of the two telephones to the 

 same period, which should then be as nearly as possible identical with the 

 period of the source of intermittent or alternating current. As an earth 

 inductor or a small magneto inductor (single magnet rotating in a flat coil) 

 would have to be used in the latter case, it seemed best to convert the appa- 

 ratus into a bifilar vibrator as shown in 

 figures 70 and 71 in elevation and plan. 

 Here mm' is a strip of thin mirror plate- 

 glass, about 32 cm. long, i cm. broad, 

 and 2 mm. thick, horizontal and in 

 a position to receive the rays NN' of 

 the interferometer (compare fig. 66). 

 Motion of mm, parallel to itself, fore 

 and aft, will therefore produce no 

 effect on the fringes ; but any rotation 

 around a vertical axis will be immedi- 

 ately apparent, as indicated in the 

 above methods for small angles. 



This strip of glass is supported by 

 the bifilar system ee, e'e', made of a 

 single thin wire of brass, 0.2 mm. in 

 diameter. The ends of ee' are wound 

 around the horizontal screws a, b, which 

 rotate with friction and are supplied with an index and scale n,n' , so that 

 any tension may be imparted to the wire. This passes below under the 

 pulleys c,d, as nearly free from friction as possible, with the object of secur- 

 ing the same tension throughout ee'. Flat clamps//', of fiber and screws,, 

 attach the strip mm to the wire at any height, but necessarily near the 

 middle of the vertical threads, where it receives the rays NN'. 



The telephonic system consists of the soft-iron horizontal screws hh', 

 similarly attached to m m' by the flat fiber clamps g, g' and the telephones 

 /,/' (omitted in fig. 70). These were made of large flat files, each provided 

 near its end with an appropriate bobbin, kk' , of fine telephone wire, the ends 

 of which are attached to clamps, as already shown in figure 66, with one of the 

 telephones provided with a commutator for reversing its current. The re- 

 sistance of each bobbin was about 140 ohms. The screws //' are used to ap- 



