CHAPTER VIII. 



ON THE TORSIONAL MEASUREMENT OF VARIATIONS OF THE ACCEL- 

 ERATION OF GRAVITY, BY INTERFERENCES. 



66. Apparatus. These experiments were undertaken at the suggestion of 

 Dr. R. S. Woodward, in order to find how far interferometer methods might 

 contribute to the measurement of g, under circumstances in which the pendu- 

 lum is inapplicable. The apparatus, as a whole, is a horizontal torsion balance 

 with the deflection readable in terms of the displacement of the achromatic 

 interference fringes. The method of registry developed would be applicable in 

 case of an ordinary chemical balance. 



In figure 100, M, M, N, N' are the mirrors of the interferometer (N f being 

 on a micrometer P, normal to its face), the white light coming from a collimator 

 at L and observed by the telescope at T. CMM'C' is the balance beam, sup- 

 ported by the tense horizontal torsion wire w, normal to the beam, mm' being 

 the auxiliary mirrors of the interferometer. This beam is of steel, U-shaped in 

 section, figure 101, and the wire ww' passing through two notches at the top 

 edges of the beam, normally to its length, is looped through a hole in the 

 bottom. One end of ww' is wound around the threads of a screw, so that ww' 

 may be kept tense at pleasure; the other end is fastened to a torsion-head, 

 the index of which is registered as to position by the graduated circular scale 

 55, coaxial with the wire. 



The ends of the beam mm' are filed flat and bent L-shaped and at their ends 

 carry two vertical needle-points, as shown in figure 100. On these rest the light 

 caps c, c', from which depend the light disks v, v', like the pans of an ordinary 

 balance. To give the system the necessary damping, the disks v, v' are loosely 

 inclosed by the cylindrical capsules or dash pots d, d r , free from contact. 



The whole balancing system is inclosed by the case CC f , the top face of which, 

 g, is plate glass. Vertical slots 5 in the long sides of the case serve for the admis- 

 sion of the torsion-wire w. These are afterwards nearly covered by glass 

 plates. The ends of CC are perforated with wide holes, also to be closed by 

 glass plates ee f . It is through these windows that the weight to be measured is 

 introduced, by placing it either on the vane v or v'. To do this smoothly the 

 weight is of wire bent along the edges of a tetrahedron. It may thus always be 

 placed or picked up and removed with the aid of a small rod or hook. 



As this weight is constant, its equivalent torsion may be registered (fig. 102) 

 by two micrometer tangent screws /, t', on the graduated circle 5, which serve 

 as stops to the index a of the torsion-head. Smaller changes of weight may, 

 therefore, be registered either by the micrometers t and t', which modify the 

 torsion, or at N', which displaces the fringes. 



The whole apparatus was erected on a heavy cast-iron base about a foot in 

 diameter. The collimator must be attached to this if a line across the slit is to 

 74 



