EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 9 



the collimator, the lamp being at A, on a separate stand. In fig. 7, the tall 

 standard A ABB of i-inch brass pipe, well braced (not shown), supports both 

 the case abcf of tin plate and the pivot supports de of the horizontal pendulum 

 HPH'P'. The rear and sides of the case are rigidly fixed, but the front may 

 be removed as a whole. Similarly, the square boxes abcf, of which a and c 

 are provided with glass plates, slide out horizontally or vertically, both in 

 front and in the rear. The pendulum is thus easily accessible for adjustment. 

 The pivots may be revolved around a horizontal axis, or moved fore and aft, 

 right and left, or up and down. The fore-and-aft movement is provided 

 with a screw adjustment, like the right-and-left movement. The method 

 of attachment is much the same as that to be described below. 



Pendulum and case are quite independent of the truss (fig. 6) , a very essen- 

 tial condition, as the truss must often be touched for optical adjustment. In 

 a later adjustment the case also was mounted in complete independence of 

 the pendulum. 



The present symmetrical horizontal pendulum was made of ^4 -inch alumi- 

 num tubing, the vertical brace PP of fo -inch aluminum tubing. The junc- 

 tions are brass tubing and the cups and slots for pivots either jeweled or of 

 glass-hard steel. There is a slot for the d pivot and a conical hollow for the 

 e pivot. Inasmuch as the horizontal pendulum is invariably under tremor, 

 with the consequent absence of static friction, the pivot e in the first experi- 

 ment was adjusted vertically, though the means were at hand for adjusting 

 it in any inclined direction, as will presently be shown. The horizontal brace 

 gh is of tense brass wire, forming a rhombus when seen from above, the object 

 being to enhance the lateral rigidity of the pendulum. Finally, the plane 

 parallel glass plates, H, H', lie to the front of the pendulum, so that a beam 

 of light may pass across, parallel to its plane, as called for in some of the 

 interferometer measurements. They are below the line of horizontal symmetry 

 and each is adjustable around a horizontal and a vertical axis. 



The case at / was specially adapted for the installation of a float K, which 

 will be described presently and was not used in the first experiments. The 

 vat C of the float must be supported on an entirely separate standard (not 

 shown) . 



5. Observations. The total weight of the apparatus, including the mirrors, 

 was M= 740 grams, with the center of gravity at /i= 13.9 cm. from the axis; 

 the effective length of the arm was R = 59.5 cm. and the period T = 2g seconds. 



The moment of inertia for an axis through the center of gravity was found 

 to be 1.51 X io 6 . Since the mass was 740 grams, this is equivalent to a radius 

 of gyration 20 = 45.2 cm. Hence, since the distance of the center of gravity 

 from the pivotal axis is h = 13.9 cm., the radius of gyration for the same axis 

 will be 4 = 47.3 cm - From this and the above period, 



^ *2 



f j- = 7.7iXio~ 3 radians, 

 1 gn 



