**Gauss meter is a very important magnetic testing instrument, so where there is magnetism, there will be a Gauss meter.**

1. Magnetic induction is a physical quantity used to describe the properties of a magnetic field. It is represented by B. The direction of B at a certain point in the magnetic field is the direction of the magnetic field at that point, and the magnitude of B represents the strength of the magnetic field at that point.

2. Magnetic field lines, magnetic flux and magnetic flux continuity theorem

**We use magnetic field lines to visually describe the magnetic field. The magnetic field lines of various magnetic fields generated by the current are shown in Figure 1. The magnetic field lines are closed lines with no head and tail around the current. The direction of the current and the direction of the magnetic field lines conform to the right-hand rule.**

3. Using the Ampere’s loop law, we can easily calculate the magnetic field produced by a current with a certain spatial symmetry. For example, calculate the magnetic field strength at point P inside a uniformly tightly wound toroidal solenoid. The concentric circles with radius r after P point are taken as a closed integral curve.

4. The law of electromagnetic induction

The law of electromagnetic induction describes the relationship between induced electromotive force and magnetic flux changes. The law states: When the magnetic flux Φ passing through a circuit changes for any reason, the induced electromotive force generated in the circuit is:

e=-dΦ/dt(12)

If the loop is composed of N turns of coils, then when the magnetic flux changes, each turn will produce an induced electromotive force, and the total induced electromotive force is equal to the sum of the induced electromotive force of each turn. When the magnetic flux passing through each turn is the same, then:

e=N×dΦ/dt(13)

The law of electromagnetic induction is one of the most commonly used laws in magnetic measurement.

When the magnetic flux in formula (13) changes periodically according to the sine law, the relationship between the effective value of the induced electromotive force and the maximum value of the magnetic flux can be derived as:

U=4.44×f×N×Φm(14)

We stipulate that the tangent direction of any point of the magnetic field lines is the direction of the magnetic field (that is, B) at that point, and the number of magnetic field lines per unit area perpendicular to the B vector is equal to the magnitude of the B vector at that point. In other words, where the magnetic field is strong, the magnetic field lines are denser, and where the magnetic field is weak, the magnetic field lines are sparse.

The total number of lines of magnetic force passing through a certain curved surface is called the magnetic flux passing through the curved surface and is represented by Φ. Take the area element on the curved surface. The normal direction and the direction of the point B form an angle θ. The magnetic flux of the element passing through the area is:

dφ=B×cosθ×ds(2)

So the total magnetic flux of S through the curved surface is:

φ=∮B×cosθ×ds(3)

When B is uniform and S is plane and perpendicular to B, the magnetic flux passing through the S plane is:

φ=B×S(4)

The law of electromagnetic induction is as shown above. We know that this is a relationship often used in magnetic measurement.