92 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



place. What is the ratio of the intensities of the horizontal com- 

 ponent of the earth's magnetic field at the two places, and at 

 which place is it the more intense ? Ans. 1.367. Field more 

 intense at first place. 



78. Two flat -ended poles arranged as shown in Fig. 46 are 

 observed to pull towards each other with a force of 1,500 pounds. 

 The steel rods are round with a diameter of 3 inches, (a) Find 

 the intensity of the magnetic field in the region between the flat 

 poles in gausses. () Find the total strength of each pole. Ans. 

 (a) 19,170 gausses, (fy 69,570 units pole. 



Note. Equation (t) on page 78 expresses that part of the force action between 

 the two poles in Fig. 46 which depends upon the polarity of the rods alone. If 

 the field between the ends of the rods is due in part to the direct action of the 

 magnetizing coil, then the force of attraction between the two rods becomes 

 sjSir X (//i 2 + 2//i7/ 2 -f -#2 2 )> wnere H\ is tne fi^d due to the magnetic polarity on 

 the ends of the rods, and /, is the field due to the direct action of the magnetizing 

 coils. Therefore, this total force consists of three parts, namely, sff^jSTr, 2sff t ffJ^tr t 

 and sffffiir. The first of these three parts is the force of attraction of the magnetic 

 poles on the ends of the rods, and the second and third parts are forces which act in 

 part upon the iron and in part upon the coils of wire which are wound upon the iron. 



79. Find the total magnetic energy in the room specified in 

 problem 69. Ans. 1,377,000 ergs. 



