499 



XIX. — Experimental Researches on the Lifting Power of the Electro-Magnet. 

 Part II. Temperature Correction ; Effects of Spirals and Helices. Bg the 

 Rev. T. R. Robinson, D. D., President of the Bogal Irish Academg, ami 

 Af ember of other Scientific Societies. 



Read June 26, 1854. 



In my former communication on the subject, I examined the relation between 

 the hfting power of the elcctro-niagnet and the force of the current which 

 excites it ; and shewed that tlie first increases much moi-e slowly than the 

 second, so that it cannot pass a limit which depends on the size of the magnet 

 by any assignable amount of current force. But besides the magnitude of that 

 force, the magnet's power depends even more on the number and distribution 

 of the spires of its helices ; we can dispose of a very restricted amoimt of 

 current. The most advantageous mode of employing a given battery is when 



its internal and external resistance are equal, its action therefore = —p. This 



for the Grove's which I use, exposing 19 inches of platinum, is 6 of my units ; 

 and for my Callan's of 90 square inches is 14.5, the last of which would only 

 excite my magnet with a single spire to one-sixtieth of its maximum. But how- 

 ever we increase the number of spires, they have all an exciting power ; and if 

 this acted equally for each on the magnet, the effect of the current might be 

 increased without limit. It is true that the increased resistance would require 

 a larger battery, but this can always be commanded. But this equality of action 

 does not exist ; the exterior spires act more feebly on account of their greater 

 distance, and those at a distance from the polar surfaces exert little influence 

 on them, both from distance and obliquity of force ; and secondly, thougli they 

 do excite fully the parts near their plane, yet the magnetism developed there is 



VOL. XXII. 3 T 



