THE MAGNETIC MEASUREMENT OF CURRENT. 35 



galvanometer is suspended midway between the coils as shown. 

 The horizontal intensity H' of the earth's magnetic field is equal 

 to o.i 8 gauss. The coils A and B each consist of one turn of 

 wire. Find the deflection of the needle which is produced by a 

 current of 0.12 abampere flowing in each coil. Ans. 16 40'. 



axis 



U-^-< 



Fig. p3. 



4. A thin brass tube 2 inches in diameter and 6 feet long is 

 wound with 1,400 turns of wire. Calculate the field intensity 

 inside of this coil when a current of 5 amperes flows through the 

 wire. Ans. 48.1 gausses. 



5. A long straight wire carrying a current of 10 amperes is 

 placed horizontally and magnetic north and south at a distance 

 of 40 centimeters directly above a short compass needle. Find 

 the deflection of the compass needle, the horizontal intensity of the 

 earth's field at the compass needle being 0.18 gauss. Ans. 29 3'. 



6. The two straight parallel wires of an electric light pole-line 

 are 20 inches apart center to center, and a current of 500 amperes 

 is flowing out in one wire and back in the other. Find: (a) 

 The intensity of the magnetic field due to the wires at a point 

 midway between them, and (&) the intensity of the magnetic 

 field due to the two wires at a point which is 20 inches from the 

 axis of one wire and 40 inches from the axis of the other wire. 

 Ans. (a) 7.88 gausses. (6) 1.97 gausses. 



