A solenoid is a closely packed helical coil constructed by winding a long conducting wire, as shown in Figure 4.1. The net field inside the solenoid is equal to the vector sum of the magnetic fields generated by each turn of the wire. If a solenoid were infinitely long, we would have an ideal configuration, in which the magnetic field inside the solenoid is uniform and parallel to its axis. A real solenoid, which is of finite length, should be a good approximation of the ideal situation, provided that we avoid making measurements close to the ends.