PROBLEMS. 449 



the north pole of one rod attracts the south pole of the other 

 rod when the poles are at an approximate distance of 50 centi- 

 meters from each other. Ans. (a) 3,200 units pole; (b) 4,096 

 dynes, 0.00924 pounds. 



NOTE. A slim rod of the best quality of hardened magnet steel retains a per- 

 manent magnetization of about 800 units pole per square centimeter sectional area. 



7. One of the magnets of problem 6 is supported horizontally 

 at its center of gravity. Find how far from the center a ten- 

 gram weight must be placed to balance the tilting action of the 

 vertical component of the earth's field, the value of which is 0.52 

 gauss ; the acceleration of gravity is 980 centimeters per second 

 per second. Ans. 8.49 centimeters. 



8. Calculate the direction and intensity of the magnetic field 

 due to both poles of one of the magnets specified in problem 6 

 at a point which is 40 centimeters from one pole and 30 centi- 

 meters from the other pole. Ans. Intensity 4.079 gausses ; makes 

 an angle of 23 27' with axis of magnet 



9. The earth's magnetic field at a certain place dips 63 below 

 the horizon and its intensity is 0.58 gauss. A room 5x6x3 

 meters high stands with its two 3x6 meter walls in the mag- 

 netic meridian, (a) Find the magnetic flux into the room across 

 its ceiling and out of the room across its floor. (&) Find the 

 magnetic flux into the room across its south wall and out of the 

 room across its north wall. Ans. (a) 155,040 maxwells, (&) 39,- 

 495 maxwells. 



10. One of the pole faces of a dynamo has an area of 20 

 cm. x 30 cm. and 1,800,000 lines (maxwells) of flux pass from 

 the pole face into the smooth core of the armature. What is the 

 intensity of the magnetic field in the air gap between the pole 

 face and the armature core? Ans. 3,000 gausses. 



11. Calculate the magnetic flux which passes out from a north 

 pole of one of the magnets specified in problem 6. Ans. 40,210 

 maxwells. 



12. The armature of a dynamo has a length, under the pole 

 face, of 30 centimeters. The magnetic field intensity in the gap 



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