Dr. Tj'ndall on the Laws of Magnetism. 



We thus see that the so-called ' lifting power ' of the magnet, 

 when the caiises of disturbance which have hitherto hampered 

 all inquiries on this subject are removed, and a delicate mode of 

 experiment is adopted, obeys a law of extreme simplicity*. 



§3. 



14. The order of the propositions at the commencement is 

 that which I thought best calculated to convey an idea of the 

 nature of the investigation. It will be shown, howevei', that the 

 third and fom-th propositions once solved, the second may be 

 derived from them as a corollary ; I shall therefore pass on to 

 the consideration of — 



Peoposition III. — A constant force beinc/ applied to a mass of 

 soft iron in a direction opposed to the pull of the magnet, to deter- 

 mine the conditions of equilibrium between this force and magnetism 

 when the distance between the magnet and the mass varies. 



15. A sheet of veiy thin foreign post paper was cut into a 

 number of small squares. By measurement with the sphereo- 

 meter, the paper was fovmd to be y^^^^ dth of an inch in thickness. 

 One of these squares was laid upon the flat pole ; the magnet 

 was excited, and the iron ball brought down upon it. A weight 

 of 100 grammes was placed upon the opposite scale-pan, the 

 handle of the rheostat was slowly turned till the ball gave way, 

 and the angle of the tangent galvanometer was then read off. A 

 second leaf was laid upon the former, the rheostat was brought 

 back to its original position, the ball was brought down upon 

 the paper, and exactly the same process was repeated. The 

 weight on the scale-pan i-emained constant dm'ing the entire 



* The method of pro\'ing the strength of a magnet before the year 1780, 

 when Coulomb published his researches, which method consisted simply in 

 ascertaining how much the magnet was able to lift, appears from the above 

 to be essentially coiTcct. 



