File:Bulletin - United States National Museum (1966) (20499566302).jpg

Title: Bulletin - United States National Museum
Identifier: bulletinunitedst2401966unit (find matches)
Year: 1877 (1870s)
Authors: United States National Museum; Smithsonian Institution; United States. Dept. of the Interior
Subjects: Science
Publisher: Washington : Smithsonian Institution Press, (etc. ); for sale by the Supt. of Docs. , U. S. Govt Print. Off.
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Smithsonian Libraries

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Figure 25 Mendenhall's 'i-\n iter i'j-second) apparatus. Shown on the left is the flash apparatus and, on the right, the vacuum chamber within which the pendulum is swung. 1 he flash apparatus consists of a kerosene lantern and a telescope, mounted on a box containing an electromagnetically operated shutter. 1 he operation of the shutter is con- trolled by a chronograph (not shown), so that ii emits a slit of light at regular intervals. 1 he nil-scope is focused on two mirrors within the apparatus, one fixed, the other attached to the top of the pendulum. It is used to observe the reflection of the flash mirrors. When the two iHln uons an- aligned, a "c oincideni e" is marked on the chrono- graph tape. 1 he second telescope attached to ili<- botto 1 the vacuum 1 hamber is for observing the amplitude of tin- pendulum swing. .m electromagnet whose coils were connected with a chronometer circuit and whose armature carried a long arm that moved two shutters, in both of which were horizontal slits of the same size. The shutters were behind the front face of the box, which also had a horizontal slit. A flash of light from an oil lamp or an electric spark was emitted from the box when the circuit was broken, but not when it was closed. \\ lien the circuit was broken a spring caused the arm to rise, and the shutters were actuated so that the three slits came into line and a flash of light was emitted. A small circular milTOr was set in each side oi the pendulum head, so that from cither face of the pendu- lum the image of the illuminated slit could be re- flected into the field of the observiim telescope. \ similar mirror was placed parallel to these two mirrors and rigidly attached to the support. The chronom- eter signals broke the circuit, causing the three siits momentarily to be in line, and when thi the slit in the two mirrors coincided, a coincidence was observed. A coincidence occurred whenever the pendulum gained or lost one oscillation on thi PAPER 44: DEVELOPMENT OF GRAVITY PENDULUMS IN THE l'lll CENTURY 333

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