86 ELEMENTARY LESSONS ON [CHAP, n 



at the same distance would repel C, then we should say 

 that the " strength " of A was twice that of B. Another 

 way of putting the matter is to say that the " strength " 

 of a pole is the amount of free magnetism at that pole. 

 By adopting the unit of strength of magnet poles as 

 defined in Art. 125, we can express the strength of any 

 pole in numbers as so many " units " of strength. 



1O3. Lifting Power. The lifting power of a magnet 

 (also called its "portative force ") depends both upon 

 the form of the magnet and on its magnetic strength. A 

 horse-shoe magnet will lift a load three or four times as 

 great as a bar magnet of the same weight will lift. The 

 lifting power is greater if the area of contact between the 

 poles and the armature is increased. Also the lifting 

 power of a magnet grows in a very curious and unex- 

 plained way by gradually increasing the load on its 

 armature day by day until it bears a load which at the 

 outset it could not have done. Nevertheless, if the load 

 is so increased that the armature is torn off, the power 

 of the magnet falls at once to its original value. The 

 attraction between a powerful electro-magnet and its 

 armature may amount to 200 Ibs. per square inch, or 

 14,000 grammes per square centimetre. Small magnets 

 lift a greater load in proportion to their own weight than 

 large ones. 1 A good steel horse-shoe magnet weighing 

 itself one pound ought to lift twenty 'pounds' weight. 

 Sir Isaac Newton is said to have possessed a little lode- 

 stone mounted in a signet ring which would lift a piece 

 of iron 200 times its own weight. 



1 Bernoulli! gave the following rule for finding the lifting-power p of a 

 magnet whose weight was w : 



where a is a constant depending on the goodness of the steel and the method 

 of magnetising it. In the best steel magnets made at Haarlem by V. 

 Wetteren this coefficient was from 19 '5 to 23. In Breguet's magnets, made 

 from Alvarre steel, the value is equally high. 



