88] MAXWELL'S TOP. 269 



mass is constantly kept at the point of support, a sharp steel point 

 turning in a sapphire cup. Maxwell's ingenious device for the 

 observation of the motion of the invariable axis, is the disk, divided 

 into four colored segments, attached to the axis of figure. The 

 colors chosen, red, blue, yellow and green, combine into a neutral 

 gray when the top is revolving rapidly about the axis of figure. If 

 however the top revolves about a line passing through a point in 

 the red sector, there will be in the center a circle of red, the 

 diameter of which is greater as the axis is farther from the center 

 of the disk and the boundaries of the red sector. Thus the center 

 of the gray disk changes from one color to another as the pole 

 moves about in the body, and by following the changes of color we 

 can study the motion. By noticing the order of the succession of 

 colors we can determine whether the axis of figure coincides most 

 nearly with the axis of greatest or least inertia, and by changing 

 the adjustments we may make it a principal axis, which is known 

 by the disappearance of wabbling, or we may make it deviate by 

 any desired amount from a principal axis. If the deviation is great, 

 and the top spun about the axis of figure, and then left to itself, 

 the top will wabble to a startling amount, but eventually the pole 

 will reach its first position and the wabbling will cease, to be repeated 

 periodically. The recovery of the top from its apparantly lawless 

 gyrations is very striking. If the adjustment is such as to make the 

 axis of figure lie near the mean axis of inertia, the top will 

 not recover, but must be stopped in its motion before striking its 

 support. 



The path of the invariable axis has been made visible by 

 Mr. G. F. C. Searle, of the Cavendish Laboratory, Cambridge, by 

 attaching to the axis of figure a card, upon which ink was projected 

 from an electrified jet. Acting upon this suggestion, the author 

 attached to the top a disk of smoked paper, upon which a steel 

 stylus, playing easily in a vertical support (shown in Fig. 82 lying on the 

 table) could write with very slight friction. One easily finds by looking 

 at the disk in its gyrations a point which remains fixed, and by applying 

 the stylus to this point, holding it on a proper support, the path of the 

 invariable axis is drawn, and found to be an ellipse or hyperbola. If the 

 stylus is not held exactly on the invariable axis, small loops are formed, 

 which enable us to count the number of turns of the top in going 

 around the polhode, and thus to verify the theory. The results of 

 several spins are shown in Fig. 83, reproduced from actual traces. 



The loops are turned out if the principal axis at the center of 

 the ellipse is that if greatest inertia, and in if it is the least, for the 

 reason that in the former case the invariable axis and the herpolhode 

 cone lie within the polhode (Fig. 83 a), while in the latter they lie 



