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THE POPULAR SCIENCE MONTHLY 



a body, and let the arrow T represent a torque acting upon the body. 

 Then the arrow AS represents the amount of spin produced by T 

 during a short interval of time, and the diagonal arrow S' represents 

 the actual spin of the body after the torque T has acted for a short 

 interval of time. 



(c) The case where the axis of torque is always at right angles to 

 the axis of spin is the most important case, and this case is exemplified 

 in the ordinary gyrostat, which is a wheel and axle supported in a 



Fig. 5. 



Fig. 6. 



movable frame as shown in Fig. 7. By talcing hold of the frame it is 

 impossible to exert a torque upon the wheel except about an axis per- 

 pendicular to OS, friction at pivots being ignored. If the frame be 

 suspended by a string as shown in Fig. 7 (side view), the pull of the 

 earth combined with the pull of the string constitutes a torque as indi- 

 cated by the arrow T in Fig. 7 (top view). The effect of this torque 

 during a short interval of time is to produce a certain amount of spin, 

 or spin-momentum, AS about T as an axis, and the resultant axis of spin 



S frame 



frame 



diagram 

 (top view) 



top view 



side view 



Fig. 7. 



becomes S' as shown in the diagram Fig. 7. The unbalanced torque 

 T, due to the weight of the wheel and frame in Fig. 7, causes the frame 

 and wheel to sweep round and round in a horizontal plane about the 

 supporting string as an axis. This kind of motion of an axis of spin 

 due to a torque which is at each instant at right angles to the axis of 

 spin is called precession, and the axis PP, Fig. 7, about which the 

 axis of spin rotates is called the axis of precession. 



