MAGNETIC EFFECT OF ELECTRIC CURRENT. 115 



two wires at a point which is 21 inches from the axis of one wire and 28 inches from 

 the axis of the other wire. Ans. (a) 4.5 gausses. (l>) 2.34 gausses. 



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

 wound with 1,400 turns of wire, (a) Calculate the field intensity 

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

 wire. (&) Calculate the total energy of the magnetic field inside 

 of the coil. Ans. (a) 48.1 gausses, (b) 341,220 ergs. 



86. A tangent galvanometer gives a deflection of 10 for 1.2 

 amperes. Calculate the deflection which will be produced by 1 5 

 amperes. Ans. 65 35'. 



87. A rectangular frame 25 x 40 cm. 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 cm. 



88. 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) acts upon the coil and specify the direc- 

 tion of the axis about which this torque is exerted. () Calcu- 

 late the torque in dyne centimeters with which the vertical com- 

 ponent of the earth's field (0.68) 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-centimeters, (b) Axis, north 

 and south; torque, 20,508 dyne-centimeters. 



89. A circular coil 10 cm. in diameter, having 50 turns of wire, is hung by a phos- 

 phor-bronze wire at the center of a large circular coil 120 cm. 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 



