THE FOUCAULT PENDULUM 447 



minutes in duration, and was continued for two working days. It was 

 necessary to measure the swing-amplitude at the beginning and end of 

 each group and it was soon noticed that the horizontal scale behind the 

 pendulum had to be shifted clockwise an apparently constant amount 

 every twenty minutes, to bring it again into parallelism with the vibration- 

 plane. It became apparent that a heavy spherical pendulum, properly sus- 

 pended and vibrating in a very small arc, was quite a different instrument as 

 to constancy of behavior from the same pendulum swinging in a large arc. 

 The observation led me, after the work for ' g' was over, to endeavor to 

 make the Foucault pendulmn a reliable lecture-room appliance. 



First, as to the point of suspension, that which was used for the Bessel 

 experiment proved entirely satisfactory and was much simpler than the 

 devices I had used previously and indeed than that used by Foucault. 

 The fine suspending piano wire was simply passed through a hole of the 

 same diameter in a horizontal metal plate firmly secured, and soldered 

 there. 



The suspending wire was the finest steel wire which would certainly 

 support the 'bob.' The 'bob' may be a lead sphere of 10 cm. diameter. 

 In the lecture-room last used, a length of 5 metres for the pendulimi was 

 easily obtained. Thus a swing of 17 cm. meant a vib-amplitude less 

 than 1°. 



In all our previous trials, the trace of the plane of the swing was made 

 by a needle attached to the bottom of the leaden sphere and made, if 

 possible, to coincide with the prolongation of the suspending wire. This 

 is troublesome to effect and maintain. If slightly in error, any rotation 

 of the bob is fatal to accuracy. It was seen that a needle is both un- 

 necessary and inconvenient. The study of the plane of vibration was 

 transferred from the needle to the suspending wire, which is unafi'ected 

 by any rotation of the bob, and describes the very surface we have to study. 



Next, it was essential to make the change of the vibration-plane visible 

 to the largest assembly, each spectator with his watch, being able to re- 

 cognize and measure the Foucault deviation. This required the introduction 

 of a lantern and screen. 



Finally, the impracticability of insulating the point of suspension and 

 the recording apparatus from disturbance in an ordinary occupied lecture- 

 room, made it essential that the Foucault motion should be so greatly 

 magnified in the spot of fight on the screen, that the whole experiment 

 could be brought within a few minutes. In this condition even a hundred 

 young fellows can be induced to keep entirely quiet for the short interval 

 required, and no other insulation is needed. 



