46 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



grams, not differing much from the above. If lightness is an object (small 

 torques being in question), the clutch and grating holder should also be made 

 of aluminum and a lighter grating attached ; but this is a secondary considera- 

 tion here, though the mass might easily be brought down to about 700 grams. 



The distance of the center of gravity from the axis was ^ = 93.1 cm.; dis- 

 tance of line of light at grating from the axis, R = no cm. The period was 

 about as above, T= 19 sec. The distance apart of the pivots, = 97.1 cm. 



Though, in general, the aluminum triangle was a copy of the steel triangle, 

 some improvements in construction were introduced. Thus a micrometer 

 attachment was added to the lower pivot, so that a direct value of <p, the incli- 

 nation of the pendulum axis, could be obtained. Windows were put in the 

 case and both pivots were now accessible without removing it. The micrometer 

 did not work as well as was expected, for reasons which did not appear. In 

 several series of experiments, the mean of the horizontal angle corresponding 

 to 30 of rotation of the micrometer screw of 32 threads to the inch and a 

 distance of 97.1 cm. between pivots was 



6-5 



6= ~- =0.00793 radian 

 820 



since the reflected spot of light traveled 6.5 cm., when the scale distance was 

 410 cm., for each step of 30 of the micrometer screw. The corresponding 

 change of inclination of the pendulum axis would correspond to one-twelfth 

 of the pitch of the screw and would be 



= 68.1X10-' radian 



32 12 97.1 



Since a <f>d, 



68. i X io- 6 

 7.93X10-* 



not differing much from the corresponding value in case of the steel pendulum 

 and there found by oscillation measurements, the pivots having been replaced 

 as nearly as possible in their former positions. The projected horizontal dis- 

 tance apart of the pivots is thus about 



97.iX8.6Xio- 3 = o.83 cm. 



which could easily be decreased and the pendulum made more sensitive (pos- 

 sibly ten times). Moreover, by using a fine wire plumb-bob, the angle <p could 

 even be roughly measured by a Fraunhofer micrometer, showing the distance 

 between plumb-line suspended from the point of the upper pivot and point of 

 the lower pivot. 



A "single-plate" film grating (see 60) was mounted at the apex of the 

 pendulum triangle. The interference rings were quite strong and clear and 

 found without difficulty. At the outset it is possible that some yield of metallic 

 parts may be registered, though the yield of the aluminum tubing, being in 

 the plane of the pendulum, should not affect its reading appreciably. 



