88 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



Weber's theory of diamagnetistn. A mass of copper near the end of an iron rod 

 has electric currents induced in it when the iron rod is suddenly magnetized, and as 

 long as this current continues to flow in the copper, the copper is strongly repelled by 

 the magnet, the lines of force from the magnet tend to spread out and pass around 

 the copper. The electrical resistance of the copper, however, very soon stops the 

 induced current and then the strong repulsion ceases. The diamagnetic property of a 

 substance has been explained by Weber on the hypothesis that the molecules of the 

 substance are perfect electrical conductors so that permanent electrical currents are 

 induced in the molecules when the substance is brought near a magnet. 



PROBLEMS. 



60. Two permanent magnets I centimeter x J centimeter X 30 

 centimeters long are magnetized to an intensity of 700 units pole 

 per square centimeter of sectional area, (a) Calculate the strength 

 of each pole. () Calculate the force with which the north pole 

 of one rod attracts the south pole of the other rod when the poles 

 are at an approximate distance of I o centimeters from each other. 

 Ans. (a) 350 units pole, (b) 1,225 dynes. 



Note. In this and the succeeding problems assume the poles of the magnet to be 

 concentrated at the center of the ends of the bars. The intensity of magnetization of 

 an iron rod is the strength of pole on one end divided by the sectional area of the rod. 

 See Art. 47. 



61. The two magnets specified in problem 60 are arranged as 

 shown in Fig. 5 1 . Find the total force with which one magnet 

 acts upon the other. Ans. 227.39 dynes (attraction). 



y . y 



S _ N S _ N [ "'f. 



\2Q cm. 



=3-*- 



Fig. 51. Fig. 52. 



62. The two magnets specified in problem 60 are arranged as 

 shown in Fig. 52. Find the total force with which one magnet 

 acts on the other. Ans. + 507.8 dynes (repulsion). 



63. A magnet I by % by 40 centimeters long having 800 units 

 pole per square centimeter of sectional area is laid across one of 

 the magnets specified in problem 60, as shown in Fig. 53. Find 



