THE CHARACTERISTICS OF INDUCED CURRENTS 5 
pictured as consisting of " lines of force " each of which 
passes lengthwise through the primary coil, and, ex- 
tending a greater or less distance from it into space at 
either end, curves outward and back so that the two 
ends meet, making each " line of force " a closed ellipse. 
The lines of force are very numerous near the primary 
coil, but become less and less frequent as the distance 
from the coil increases. The number of lines of force 
present and the distance from the coil at which they can 
be detected depend upon the intensity of the current 
flowing through the coil. 
If another coil of wire, the secondary, be placed 
within the field of force about the primary in such 
position that lines of force pass lengthwise through it, 
any alteration in the number of lines of force compre- 
hended within the secondary generates within it a cur- 
rent which is the induced current. This current, which 
depends upon changes within the field of force, ceases to 
be generated whenever the field of force becomes steady, 
and outlasts the change in the field only the brief frac- 
tion of a second required for the current to die away. 
The direction of the induced current depends upon the 
direction of the current through the primary coil, and also 
upon whether the change in the field is an increase or a 
decrease in the number of lines of force. The intensity 
of the currents induced in any secondary coil depends 
upon the number of lines of force moving through it, 
