658 Professor W. G. Adams [June 3, 



Faraday sliowed that on turning such a coil in a magnetic field a 

 current of electricity is induced in the coil, and the strength of this 

 current is proportional to the number of lines of force cut by the 

 coil. 



We may describe such an arrangement as a magneto-electric 

 machine, in which the magnet employed is the earth itself. By means 

 of this instrument we may determine either the horizontal or the 

 vertical magnetic force of the earth. By placing the axis vertical and 

 spinning the coil at a given rate we may determine the horizontal 

 force, and by placing the axis horizontal in the magnetic meridian 

 and spinning the coil at the same rate we may determine the vertical 

 force, the currents produced in the two cases being in the same ratio 

 as the numbers of the lines of force cut in the two positions. 



The greater the angle at which the axis of rotation is inclined to 

 the direction of the lines of force, the greater will be the number of 

 them included in the revolving circle and the greater the induced 

 current produced in the coil. 



Thus placing the axis in different positions we get currents of 

 different strengths, and may readily see that we get the greatest 

 current when the axis is at right angles to the direction of the lines 

 of force, i. e. to the line of the dip. 



The current produced in each half-turn of a coil of wire revolving 

 on an axis is proportional to the number of lines of force cut by the 

 coil during its rotation, so that the total current in the coil will be 

 proportional to the number of lines of force cut by the coil multiplied 

 by the number of turns of wire in the coil. 



When the axis of rotation is in the magnetic meridian, but per- 

 pendicular to the lines of magnetic force of the earth, the current in 

 that half of the coil which is moving from west to east will be from 

 north to south, and the current in the other half of the coil which is 

 moving from east to west will be from south to north, so that in the 

 whole coil we get during every half-turn a current in one direction 

 all round the coil. 



During the next half-turn we get a current all round the coil 

 in the same direction as looked at from without, i. e. in the opposite 

 direction in the coil of wire. The direction of the current in the 

 coil, as we look at it from the east, is the same as the direction of 

 rotation of the coil as we look at it from the north. 



A continuous current may be obtained from the coil by reversing 

 the connections with the ends of the coil by means of a commutator 

 at the same time as the currents are reversed in the coil. 



We may further make use of such a coil to find the direction of 

 the lines of force, for if we place the axis parallel to the lines of 

 force, the currents in opposite halves of the coil will balance one 

 another, because each line of force is cut twice by the coil, and so 

 no current is produced in the external circuit through the galvano- 

 meter. 



If, then, we place the coil so as to get no current when we rotate 



