THE FOUCAULT PENDULUM 449 



that its fulcrum C is distant from the stationary w-ire, a length equal to 

 the longer recording arm of the lever. Thus CO = CB. The end B of 

 this arm describes a circle passing through 0. Suppose the vibration- 

 plane changes from OB to OB' then the lever turns from CB to CB'. But 

 the angle BCB' = 2B0B'. Hence the lever and mirror turn twice as fast 

 as the vibration-plane. Now the spot of light turns twice as fast as the 

 mirror. Hence average angular speed of spot = 4 times the Foucault 

 speed. 



Then if the screen be graduated into 4-inch intervals and placed 370.9 

 inches from the mirror the spot of light in this place ought to describe 

 each interval in 60 seconds, and would take three minutes to describe 

 three intervals. Three minutes were found to be a convenient and suffi- 

 cient space of time for the experiment before the largest class. It is an 

 easy duration to verify with any watch having a second hand. 



Or putting the formula into a different shape, sine latitude = — -^ 



where t equals the number of seconds the spot takes to move 12 inches. 

 We find t with our watches and may compare the latitude thus found with 

 its known value. 



The push of the lever, unfortunately, is rather a blow than a pressure. 

 Though it is a very delicate blow, the great magnification of the displace- 

 ment reveals the fact that the wire at its next swing may not touch the 

 lever. This pause really does not seriously affect the result of a three- 

 minute trial. A more essential point is to keep the lever from recoiling 

 if we try to dampen its motion so as to make it correspond exactly to that 

 of the vibration-plane. After many kinds of dampening were tried, it 

 was found that the only one in which a fatal recoil or back lash was absent, 

 was the friction of a sharp steel needle point on a smooth glass surface. 

 Accordingly a fine needle was thrust through the short arm of the lever 

 so that its point might rest on a small glass plate fastened below. The 

 push of the wire on the lever merely caused this point to scrape hghtly on 

 the glass without any recoil which the spot of light could show.f 



The whole contrivance thus described was completed the year after 

 the Bessel experiment, that is in 1890, and it was then shown to the class 

 in Physics. The entire arrangement was kept, like the Atwood Machine, 

 a fixture of the lecture room. It was used regularly from 1890 to 1907, 



X Outlines of the recording lever (see fig. 4). AB the length of the straw say 

 12 cm. F the fulcrum needle resting in a small glass cup C. B is the end touched 

 by the swinging wire: m is a counterpoise balancing the lever on C. n is a small 

 needle pressing lightly on the glass plate p. GH is the wooden base. R is the plane 

 mirror. 



