70 



ADVANCED ELECTRICITY AND MAGNETISM. 



field due to the north polar edge of the strip at a point distant 

 1 8 centimeters from the edge, and specif yi ts direction. Ans. 

 8.89 gausses directed away from the edge. 



cm. 



N 



N 



N 



S. 



S 



end view 



&ide view 



Fig. 



Note. The magnetic field intensity at the point p due to the north polar edge 

 of the ribbon magnet may be found as follows: Imagine a cylinder 18 centimeters 

 in radius with its axis along the north polar edge. Let h be the field intensity 

 at the surface of this cylinder due to the north polar edge of the ribbon magnet. 

 Then h is at right angles to the cylindrical surface, and the magnetic flux through 

 one centimeter of length of the cylindrical surface can be easily found in terms of 

 h; and this flux must be equal to 4?r times the number of units pole on one centi- 

 meter of length of the north polar edge of the magnet. 



5. A vertical wire 3 meters long is moved sidewise, towards 

 magnetic east or west, at a velocity of 25 meters per second. 

 Find the electromotive force induced in the wire in volts, the 

 horizontal component of the earth's field being 0.18 gauss. Ans. 

 0.00135 volt. 



6. The pole face of a dynamo is 30 centimeters long in the 

 direction parallel to the axis of the armature, and the field inten- 

 sity in the gap space between the pole face and the armature 

 core is 6,000 gausses. The wires on the armature are 12 centi- 

 meters from the axis of the armature, and the speed of the 

 armature is 1,800 revolutions per minute. Find the electro- 

 motive force in volts induced in each armature wire (30 centi- 

 meters in length) as it passes across the pole face. Ans. 4.07 

 volts. 



