196 Dr. Tyndall on the Progress of the Physical Sciences : 



The above experiments show that the attractive force exerted 



on the svibmagnet increases with the length of the latter ; this, 



however, has its limit. To obtain a clear insight of the matter. 



Dub first of all chose a number of bars of the same diameter 



but of different lengths, and then a number of the same length 



but of different diameters. The former varied from 2 to 24 



inches in length, and were all 1 inch thick. The attracting 



magnet was also an inch thick. When a stream of 20° was sent 



round the magnet, the following weights were found necessary 



to force it and the submagnet asunder : — , , .,.- 



° r: L' r, (1,1 r •'/ 



Length of submag. 2" 3" 4" 5" 6" 9" 15" 18" 21" 24" 



Attraction 1-23 173 2-38 3 3-27 4-43 4-89 518 6-34 596 



"With a stream of 35° the following weights were obtained : — 



Length of submag. 2" 3" 4" 5" 6" 9" 15" 18" 21" 24" 

 Attraction 296 4-5 6-08 7-56 8-88 13-8 16-4 188 22 17-5 



That the increase of attraction which accompanies the length- 

 ening of the submagnet has a limit, is rendered clear from the fol- 

 lowing table, wherein are exhibited the attractive forces exerted 

 upon a number of bars of y''g-ths of an inch in diameter; the magnet, 

 as in the former case, was an inch thick and 12 inches long : — 



Length of submag. 2" 3" 4" 5" 6" 9" 12" 15" 18" 21" 24" 



Stream 20°; attract. 1-7 2-86 3-39 388 4-91 6 652 59 6-32 6-3 6-32 



... 35°; ... 4-36 6-2 774 80 92 1014 108 9-9 10-5 10-3 10-5 



We here see, that after the submagnet has attained a length 

 of about 9 inches, its attraction remains constant ; that full 

 15 inches might be cut away from the 24" bar without diminish- 

 ing the action of the magnet upon it. If the bars be thinner, 

 the point of maximum attraction exhibits itself still sooner. The 

 results contained in the following table express the action of the 

 same magnet on bars y^ths of an inch in diameter : — 



We here see that from 4 inches upwards the increase is hardly 

 worth notice : but we derive another significant conclusion from 

 the table ; and that is, that an addition to a strong stream effects 

 less than a proportionate addition to a weak stream : for example, 

 by increasing the stream from 20° to 35°, that is, by doubling 

 it, the carrying power of the magnet in the case of the 2" bar 

 is also doubled ; but by increasing the stream from 35° to 45°, 

 that is, in the ratio of 7 : 10, the cariying power is increased only 

 in the ratio of 7 : 8. 



These experiments are at variance with the law which afiirms 

 that the lifting power of an electro-magnet is proportional to the 

 square of the magnetizing stream. Casting an eye on the attrac- 



