ELEMENTARY THEORY OF ELECTROMAGNETISM. 69 



fraction depends upon the closeness of coupling and upon the 

 closeness of tuning of the primary and secondary systems.* 



PROBLEMS. 



1. A room 6 meters long by 5 meters wide by 3 meters high 

 has its longest dimension magnetic north and south. The inten- 

 sity of the earth's field in the room is 0.62 gauss and the dip is 

 72. Find the number of lines of magnetic flux across each of 

 the walls, the ceiling, and floor of the room, and specify in each 

 case whether the flux is passing out of the room or into the room. 

 Ans. East and west walls, zero; north wall, 28,740 maxwells 

 out; south wall, 28,740 maxwells in; ceiling, 176,900 maxwells 

 in; floor, 176,900 maxwells out. 



2. The pole face of the field magnet of a dynamo has an area 

 20 centimeters by 30 centimeters. The magnetic field between 

 the pole face and the armature core is perpendicular to the pole 

 face at each point and its intensity is 6,000 gausses. Calculate 

 the number of lines of force which pass from the pole face into 

 the armature core. Ans. 3,600,000 maxwells. 



3. A slim magnet 0.25 centimeter thick and 0.5 centimeter 

 wide is magnetized to an intensity of 1 ,000 units pole per square 

 centimeter of sectional area. Find the number of lines of magnetic 

 flux which emanate from the north pole of this magnet. Ans. 

 1571 maxwells or lines. 



4. A very long steel ribbon of which the thickness is O.I centi- 

 meter is magnetized so that one edge becomes a long north pole 

 and the other edge becomes a long south pole, as shown in Fig. 

 4, the intensity of magnetization being 800 units pole for each 

 square centimeter of section of the steel (80 units pole for each 

 centimeter length of edge). Find the intensity of the magnetic 



* The mathematical theory of the electrical system in Fig. 52 and of the me- 

 chanical system in Fig. 53 is identical. This theory is developed in a paper by 

 Louis Cohen, Bulletin of Bureau of Standards, Vol. 5, pages 511-541. 



