268 Mr. Joule on the Calorific Effects 



represented by fig. 3. Its length was thirty-two inches; its 

 breadth in the middle part eight inches; at the ends three 

 inches. It was bent nearly into the shape of the letter U, so 

 that the shortest distance between the poles was slightly more 

 than ten inches. 



Twenty-two strands of copper wire, each 106 yards long 

 and about one-twentieth of an inch in diameter, were now 

 bound tightly together with tape. The insulated bundle of 

 wires, weighing more than sixty pounds, was then wrapped 

 upon the iron, which had itself been previously insulated by a 

 fold of calico. Fig. 4 represents, in perspective, the electro- 

 magnet in its completed state. 



Fig. 4. 



In arranging the voltaic battery for its excitation, care was 

 taken to render the resistance to conduction of the battery 

 equal, as nearly as possible, to that of the coil, Prof. Jacobi 

 having proved that to be the most advantageous arrangement. 

 Ten of my large Daniell's cells, arranged in a series of five 

 double pairs, fulfilled this condition very well, producing a 

 magnetic energy in the iron superior to anything I had pre- 

 viously witnessed. I will mention the results of a few experi- 

 ments in order to give some definite idea of it. 



1st. The force with which a bar of iron three inches broad 

 and half an inch thick was attracted to the poles, was equal, 

 at the distance of T ] ^th of an inch, to 100 lbs; at ^th of an 

 inch to 30 lbs; at half an inch to 10^ lbs. ; and at one inch to 

 4 lbs. 13 oz.* 2nd. A small rod of iron three inches long, 

 weighing 148 grs., held vertically under one of the poles, 

 would jump through an interval of If inch; a needle three 



* The above electro-magnet being constructed for a specific purpose, 

 was not adapted for displaying itself to the best advantage in these instances. 

 On account of the extension of its poles (three inches by half an inch) many 

 of the lines of magnetic attraction were necessarily in very oblique direc- 

 tions. Theoretically, circular poles should give the greatest attraction 

 from small distances. 



