PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 119 



rid in great measure of the disturbing effects of eddy currents, one or 

 two large magnets with massive cores were useful for purposes of com- 

 parison. One of these (P), which weighs about 1500 kilograms, has 

 the general shape shown in Figure 14. The outside dimensions of the 

 frame proper are about 101 cms. X 80 cms. X 40 cms. The base is 

 of cast iron and of rectangular cross-section (20 cms. X 40 cms.), the 

 cylindrical arms are of soft steel 25 cms. in diameter, the rectangular 

 pole pieces are 4.5 cms. thick, and the area of each of the opposed 



u 



FlGCRE 12. 



This illustrates different meanings 

 of the word inductance. 



FlGCRE 13. 



faces is about 580 square centimeters. The four coils have together 

 2823 turns, and a resistance at 20° C. of about 12.4 ohms. 



Figure 15 shows in outline the electromagnet Q, which weighs about 

 300 kilograms : the core has a square cross-section of about 156 square 

 centimeters area, and is built up, cobhouse-fashion, of soft iron plates 

 about one third of a millimeter thick, each of which was immersed in 

 thin shellac and then thoroughly baked in an electric oven before it 

 was used. Each of the spools, which are practically alike, weighs about 

 30 kilograms and has four coils, an inner one forming a single layer, 

 the next forming three layers, and the two outer ones wound together 

 side by side from two supply spools, and each equivalent to five layers ; 

 in all, both spools together have 3883 turns. The whole core frame is 

 about 74 cms. long and 62 cms. broad. One stratum 2.5 cms. high 



