Lifting Power of the ElectroMagnet. 



521 



3-68 times as powerful. By theory the ratio should be 3'27 : 1. There is the 



same decrease in -— - as ^ increases, which was remarked in the other magnets. 

 Ay ° ' 



and from which the existence of a limit to L was inferred. 



In this magnet (as in the others), A is constant for every value of ^ above 

 one which is less than 600. At 200 it has considerably decreased, so that the 

 critical value is probably near 300. Few observations of \ were taken, but 

 those few seem in due proportion to the other magnets. 



The iron cylinders were then removed, and those of steel substituted. 

 These were tempered to a pale straw colour, and their polar surfaces, which 

 cast in this process, were made true by careful working with a tool made of lac 

 fused with fine corundum. The same helices were used, but as it was desirable 

 that the permanent magnetism \ should not be disturbed, the current was not 

 reversed in each observation of a set. The temperature correction was first 

 sought with 1^ = 1282-92. 



Table XXXI. 



These give 



£= 134-741 1 + 1 X Z-' (6-51193) - <= x Z-'(3-91068) j. 



I did not anticipate the remarkable difference between this correction and 

 that of the iron magnet, especially as heat is known to lessen the power of a 

 permanent magnet. As far as the base and keeper are concerned, there can be 



