64 Messrs. Ayrton, Mather, and Sumpner 



convolution, since the direction of the force at P is along the 

 magnet NP. But if the magnet be placed nearer to the con- 

 volution, as at N'P', or farther from it, as at N'T", there will 



Fig:; 3. 



be a torque exerted on it due to this convolution of radius 

 OA ; the two torques, however, will be in opposite directions. 

 Now as the radii of the greater number of convolutions con- 

 tained in the coil are greater than the half length of the 

 magnet, it is clear that the torque exerted by the convolution 

 OA when the magnet is to the right of NP is in the same 

 direction as that exerted by the coil as a whole, and that the 

 torque exerted on the magnet by this convolution when the 

 magnet is between OA and NP is opposed to the torque 

 exerted by the rest of the coil. In other words, if NP be the 

 position of the half-magnet in the coil, a convolution of 

 smaller radius than OA must not be wound in the plane that 

 passes through OAM at right angles to the paper. And if a 

 line APQ be drawn so as to cut all the lines of force at points 



