COILS, SOLENOIDS AND OTHER THINGS
Let’s examine the objects we will be using:
This is a solenoid: a conducting wire wound around a tube.

Click on the photo to see the experiment

This is a solenoid with about 100 windings in two layers. The wire is thick and, on connecting it to a 1.5 V battery, the current can be about 1A.

To see, click on the photo

When the length of a solenoid having n windings per unit of length and electrified by current I is much greater than its radius, the induction field inside it is parallel to its axis and its modulus is given by the following expression:

It is thus easy to have relatively high values by having many windings.

The fundamental Theorem of Ampere connects generic configurations of currents to the fields they generate.
The circuiting of the vector of magnetic induction generated by direct currents, along a closed line, is proportional to the algebraic sum of the currents within such a line.

Ampere’s theorem is important since it allows calculation of the field generated by currents in a circuit of different forms.

If in the solenoid we introduce a nucleus of material having magnetic permeability µ this is magnetized with a magnetic induction µ times greater. Ferromagnetic materials (iron, nickel and many others) may reach values of µ of even 1000. This is why electric motors, electromagnets and so on always have an iron nucleus.

One instrument that Ampere made much use of consisted of the magnetic needle of a compass inside a coil of isolated wire through which he sent the current to see or measure. This was the first kind of galvanometer, later known as the “tangent compass” since the current is proportional to the tangent of the needle’s angle of deflection.