EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 15 



The manner in which the jewels were secured is shown in fig. na, and 

 sectionally in fig. nb, turned at right angles around a vertical axis to the 

 preceding figure. Here a is the lower end of the central vertical tube of the 

 horizontal pendulum into which the stem b of the forked holder c fits very 

 snugly. The two flat prongs c carry the brass screw with fine thread d, which 

 is horizontal and is secured in any position by the lock-nut e. The conically 

 hollowed jewel, black in figure, is firmly embedded in the small brass cylinder 

 /, which in turn may be screwed centrally into the wider cylinder d and fixed 

 by a lock-nut. P is the brass stem carrying the needle of glass-hard steel 

 which dips in the sapphire socket. The cylinders P and / are coaxial, but 

 may be given any inclination to the vertical and then locked. As the effective 

 weight of the pendulum does not exceed 170 grams, the strain on the pin and 

 jewel is not excessive, and the results appear to show that they rendered 

 excellent service. The upper pivot played in a groove of glass-hard steel as 

 before, and it did not seem necessary to modify this. 



The remainder of the horizontal pendulum was of the form already sketched 

 in figs. 6 and 7 ; but precautions were subsequently taken to mount the water- 

 bath for the float on a separate pillar, quite independent of the horizontal 

 pendulum and its case. Later the case was also independently mounted. 

 Trial was made of a water damper attached to the end of the beam (H ' in 

 fig. 6) on the side opposite to the mirror H . This, however, was soon discarded 

 because of the capillary forces introduced. As a rule, the damping obtained 

 at the float is adequate. 



In order to set the zero of the pendulum at a given point, as well as to vary 

 the inclination of the axis by the definite amount needed in the independent 

 data of <p (see page 20), the lower pivot is virtually on a micrometer screw, 

 capable of moving it by a definite amount z at right angles to the plane of 

 the pendulum. This device is shown in fig. nc, where p is the pivot screw 

 with lock-nut securing it to the brass rod /. The latter fits snugly in the end 

 of the piece of ^4-inch gas-pipe k and is secured by the lock-nut h, the pipe 

 being longitudinally slotted within it. The rod I is firmly fastened to the 

 micrometer screw m, the nut of which, n, drags the rod / from left to right, in 

 spite of considerable friction, when turned clockwise. Finally the rotation of 

 m is measured on a dial q fixed to the pipe k. Hence, as shown below, 1 1 , 

 if a displacement z is given to the lower pivot at a distance y below the 

 upper, a = z/y = <p8. 



9. Observations. The constants of the pendulum were the same as in the 

 preceding case, with the exception of the period T, which was found to be 

 16.2 seconds. The other constants are M = 971 grams; ^ = 61.5 cm.; h 13.1 

 cm.; M V=i6g grams; ^ = 40.7 cm.; whence 



6 = 278 X io~*x; a = <pO = 6X io-*x radians, nearly. 

 To obtain 



