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 



