10 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



or about 0.44 degree. Hence, the change of inclination of the pendulum is 

 nearly 01 = 0.00770. The force at a distance ^ = 59.5 cm. from the pivotal 

 axis (place of the plate of glass or grating) is 



F R = Mgh<p&N/R = 1 1 AN, 



supposing the interferometer to be used and that AJV is the displacement 

 of the micrometer. Thus F R here and in the preceding case, when referred 

 to the same <p and A7V, shows 



F/VA/V=io8o and 4000, respectively, 



so that the present horizontal pendulum is about four times more sensitive 

 than the other if the same interference method is used. This, however, was 



not desirable at the outset, the above method of reflection admitting of easier 

 interpretation. Hence, Q = x/^D nearly, where x is the distance apart of the 

 two slit images corresponding to the distance D. 



The results obtained for the current changes of inclination, a, in seconds 

 of arc, are given in fig. 8, on the same plan as the preceding. The work was 

 continued for nearly two weeks, but only a brief example is shown in the 

 figure. 



In the present experiments the whole weight of the pendulum, 740 grams, 

 rests directly on the lower vertical steel pivot, and the friction is correspond- 

 ingly large and probably excessive. Static friction, however, was not present, 

 the pendulum being continually in motion. On November 7 certain 

 changes of apparatus were undertaken, and a new zero had to be selected. 

 As a whole, the changes of inclination of this pendulum lie within 1.5 seconds 



