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An extension of the previous case of a single current loop
is to consider a large number N of such loops
tightly packed together over a distance L - such a device
is called a solenoid. This is illustrated in Fig. 1.8.
This device has many important applications
for two reasons: one is that fairly strong magnetic fields can be produced
by using a large number of turns, and secondly, the magnetic field
is fairly constant in magnitude and direction throughout the solenoid, except
near the ends. The magnitude of the magnetic field of a solenoid is
where n = N/L is the number of turns per unit length. The direction
of the magnetic field is given by the same rule for determining the
direction of a magnetic field of a single current loop.