36 ADVANCED ELECTRICITY AND MAGNETISM. 



7. A rectangular frame 25 X 40 centimeters has 10 turns of 

 wire wound upon it. The frame is balanced horizontally upon 

 an axis pointing due magnetic east and west. A current of 28 

 amperes is sent through the wire. Required the distance from 

 the axis at which a lo-gram (9,8oo-dyne) weight must be hung to 

 balance the torque action due to the earth's magnetic field at a 

 place where its intensity is 0.57 gauss and its dip is 63. Ans. 

 0.74 centimeter. 



8. A circular coil has 100 turns of wire. The diameter of the 

 mean turn is 16 centimeters, and a current of 15 amperes flows 

 through the coil. This coil is suspended with its plane lying 

 vertical and magnetic north and south, (a) Calculate the torque 

 in dyne-centimeters with which the horizontal component of 

 the earth's field (0.2 gauss) acts upon the coil, and specify the 

 direction of the axis about which this torque is exerted, (b) 

 Calculate the torque in dyne-centimeters with which the vertical 

 component of the earth's field (0.68 gauss) acts on the coil, and 

 specify the direction of the axis about which this torque is 

 exerted. Ans. (a) Axis, vertical; torque, 6,032 dyne-centi- 

 meters, (b) Axis, north and south; torque, 20,508 dyne- 

 centimeters. 



9. A circular coil 10 centimeters in diameter, having 50 turns of 

 wire, is hung by a phosphor-bronze wire at the center of a large 

 circular coil 120 centimeters in diameter, having 500 turns of 

 wire. The suspending wire is free from twist when the planes 

 of the two coils are at right angles, and a torque of 250 dyne- 

 centimeters twists the wire through one radian of angle. How 

 much current must pass through the two coils in series to cause 

 the suspended coil to turn 30 from its position of equilibrium? 

 What happens if the current is reversed in one coil? What 

 happens if the current is reversed in both coils? Ans. 0.27 

 ampere. 



