1 14 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



The D'Arsonval galvanometer may be used for the approx- 

 imate measurement of weak currents, because the deflection, 

 within a small range, is proportional to the current ; that is 



i = kd 



in which i is the current flowing through the galvanometer, d 

 is the deflection in scale divisions, and k is a proportionality 

 factor which is called the reduction factor of the galvanometer. 



PROBLEMS. 



80. A horizontal wire 10 meters long, stretched due magnetic 

 east and west, is pushed up by the horizontal component of the 

 earth's field with a force of 2,500 dynes. What is the direction 

 and strength of the current in the wire ? The horizontal com- 

 ponent of the earth's field is 0.2 gauss. Ans. 125 amperes east. 



81. The armature of a dynamo has a length, under the pole- 

 face, of 30 cm. The magnetic field intensity between the pole- 

 face and the armature core is 6,000 gausses. The surface of the 

 armature is covered with straight wires parallel to the axis of the 

 armature. Each of these wires carries a current of 75 amperes. 

 Calculate the force acting on each wire. Ans. 1,350,000 dynes. 



82. A horizontal electric light wire stretched due magnetic 

 north and south carries 1,000 amperes of current flowing towards 

 the north. The length of the wire is 250 meters, the intensity 

 of the earth's field is 0.57 gauss and the magnetic dip is 63. 

 Find the value of the force pushing on the wire and specify its 

 direction. Ans. 1,269,500 dynes west. 



83. A circular coil of wire of 20 cm. radius has 15 turns of 

 wire. How much current is required in the coil to produce at 

 the center of the coil a field intensity of o. 57 gauss ? Ans. o. 1 2 1 

 abamperes. 



84. The two straight parallel wires of an electric light pole-line are 35 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 (l>) the intensity of the magnetic field due to the 



