454 UNIVERSITY OF VIRGINIA PUBLICATIONS 



would be preferred were the blackboard not present. The box is glazed 

 in front to avoid concealing the bob from the class, and is slotted in its 

 top parallel to the wall to permit the passage of the wire swinging in its 

 vibration plane. The width of the slot is about 1 cm., length 12 cm. 



The indicating mechanism, which is seen at the top of' figure 7 and of 

 figure 8 is supported from the box in the following manner. An inverted 

 L of half-inch round iron screwed to the back of the box rises and bends 

 forward as shown. To this is clamped a "right angled clamp with screw 

 adjustment" such as are furnished by apparatus dealers as parts of standard 

 laboratory supports. This clamp is of course available for other apparatus 

 when the pendulum is not in use. The clamp supports a short rod, on top 

 of which is fastened a wood block and to this is waxed a small glass plate 

 about 8x8 cm. forming a smooth horizontal table upon which the indicat- 

 ing lever rests and is damped according to the principle Professor Smith 

 states on p. 449. 



The lever is a tripod whose construction is clearly shown in figure 8. 

 One leg consisting of a sewing needle is stepped, point down, into a "glass 

 jewel" or glass cup (p. 449) which is waxed to the plate about 1 cm. from 

 its edge. The other two legs slide over the plate and support the mirror 

 as well as damp its motion. The forward leg is prolonged horizontally 

 and projects over the edge of the plate where its tip is touched by the 

 passing pendulum wire. (The scrap of paper stuck on the left hand cor- 

 ner of the plate is merely for the purpose of showing the extension of this 

 lever in the photograph and does not normally belong there.) It is well 

 to see that the surface of the plate possess no flaws where it is likely to 

 cause the lever to stick. A little water as a lubricant under the sliding 

 legs sometimes helps. The mirror is mounted back of the needle in such a 

 position as to insure stability. This arrangement is easy to make and 

 requires little further attention when once adjusted. 



The two glass plates pasted into an L shown in the background of 

 figure 8 is an auxiliary device for use in setting up. Its purpose is to tem- 

 porarily extend the level area of the glass plate on its front edge when the 

 pendulum is at rest so that the lever may be swung around into position 

 CO, figure 3. Then the adjustment CO = CB= length of the lever (in 

 this case 2.7 cm.) may be nicely finished by means of the screw adjustment 

 of the "right angled clamp." The angle OCB is of the order of magnitude 

 of 120°. 



. The pendulum is started by the traditional method of pulling aside 

 with a string and, when all is quiet, burning the string. The string passes 

 through a hole in the box to a hook fastened just outside. There the 



