120 



PEOCEEDINGS OF THE AMERICAN ACADEMY. 



and reaching across the middle of the core (Figure 16 a) within one of 

 the spools, is made up of five portions insulated from one another, and 

 each of these is surrounded by an exploring coil of insulated wire. 



Figure 16^ shows the form of the cross-section of the rectangular 

 core frame of a 15 kilowatt transformer (i?) constructed for experi- 

 mental purposes and belonging to the Lawrence Scientific School. 

 Besides a low-resistance primary coil, this transformer has 19 similar 

 coils each of about 85 turns, any number of which may be connected 

 to form a secondary circuit. The outside dimensions of the core frame 

 are about 78 cms. and 34 cms. ; the area of the cross-section of the 

 finely divided core is about 108 square centimeters. 



Figure 14. 



The electromagnet P. This magnet has a solid core which weighs about 1500 

 kilograms. 



Magnet S has a core consisting of two round solid pieces 76 cms. 

 long and 7.4 cms. in diameter with axes 24 cms. apart, connected 

 together at the ends (so as to form a rectangular frame) by two massive 

 iron blocks. This magnet has two spools, each of which has two coils 

 formed by winding two strands side by side; the whole number of 

 turns is 1724. 



The core of magnet T forms a square 58 cms, long on the outside 

 and 53.5 cms. wide. Its cross-section is a rectangle 7.5 cms. by 6.7 

 cms. The core is built up of sheet metal 0.38 of a millimeter thick. 



Through the kindness of Dr. George Ashley Campbell I have been 

 aJlowed to use also seven toroidal coils (of inductances between 0.3 and 

 13 henries) wound on cores made of very fine (No. 38 B. & S.) iron wire. 

 Such cores are, of course, extremely expensive, but the disturbing 



