Dr. Tyndall on the Laws of Magnetism. 275 



series of experiments, viz. 100 grammes. Now it is easy to see, 

 that the greater the distance which separates the ball from the 

 magnet, the greater must the power of the latter be to counter- 

 balance the force acting against it. We have thus the equation 



m=f{d), 



where m represents the magnetism of the core, and d the distance 

 which separates ball and magnet. It will be proved immediately 

 that this function possesses the form 



VI = n i/d, 



where re is a constant. This expressed in words announces the 

 remarkable law, that when the distance between the magnet and 

 the sphere of soft iron varies, and a constant force opposed to the 

 pull of the magnet is applied to the latter, to hold the ball in equi- 

 librium the power of the magnet must vary as the square root of 

 the distance. 



The quantity m is expressed by tan (3. The proof of the above 

 law will therefore depend on the fulfilment of the equation 



tan/3 



— yy = const. 

 vd 



In the following table the thickness of a leaf is taken as the 

 unit of distance, and tanyS is multiplied by 100=r. 



III the following series the experiments are continued to 

 greater distances. A constant weight of 200 grammes was here 

 placed upon the scale-pan. 



