A teslameter is an instrument that measures the intensity of a magnetic field, expressed in teslas (T) or, for weak fields, in microteslas (µT). On a smartphone, it is provided by the magnetometer. As a reference, the Earth’s magnetic field is about 47 µT in western Europe.
Discover FizziQ
How to measure it in class
With FizziQ, the magnetometer displays the intensity of the magnetic field and allows studying its variations.
Steps:
- Open FizziQ and display the magnetic field measurement (magnitude of the vector, in µT).
- Away from any magnet, read the Earth’s field value and check that it is of the order of a few tens of µT.
- Bring a magnet closer then move it away: observe the sharp increase then the fast decrease of the intensity.
- Measure the field at different distances from a magnet and plot the curve to reveal the 1/d³ decrease.
- Bring the phone near a wire carrying a direct current (Oersted effect) and spot the small variation of the field.
Scientific activities on this topic
Possible extensions with FizziQ: study of the decrease of a magnet’s field with distance, mapping of the Earth’s field in a room, or demonstration of the Oersted effect around a wire. See the smartphone sensors guide.
Learn more
The smartphone’s magnetometer is most often a Hall effect or magnetoresistive sensor, which measures the three components Bx, By and Bz of the magnetic field. The app computes their magnitude, that is, the total intensity of the field, independent of the device’s orientation. The sensor does not distinguish the origin of the field: it adds up the Earth’s field, that of nearby magnets and that produced by the device’s own currents. A smartphone is not a metrology instrument; its values carry a device-specific offset and non-negligible noise, but they are largely sufficient to study physical laws such as the 1/d³ decrease.
Orders of magnitude: Earth’s field in western Europe ≈ 47 µT; at the surface of a neodymium magnet, the field can exceed several hundred thousand µT (several tenths of a tesla) and saturate the sensor; around a wire carrying 1 A, the field is about 20 µT at 1 cm and only 4 µT at 5 cm. The resolution of a consumer magnetometer is of the order of 0.1 to 0.3 µT.
Formula
Around a long straight wire carrying a current, the magnetic field (Oersted effect) is:
B = µ₀ × I / (2π × d)
Where:
- B: magnetic field intensity, in teslas (T).
- µ₀ = 4π × 10⁻⁷ T·m/A: magnetic permeability of vacuum.
- I: current intensity, in amperes (A).
- d: distance to the wire, in metres (m).
For a magnet treated as a dipole, the intensity decreases as 1/d³ along its axis.
Application examples
- Measurement of the Earth’s magnetic field and its components.
- Study of the decrease of a magnet’s field with distance (1/d³ law).
- Demonstration of the Oersted effect around a conducting wire.
- Detection of ferromagnetic objects hidden in a wall.
- Comparison of the strength of different magnets.
- Locating sources of magnetic disturbance in a room (metal structures, electrical appliances).
FAQ
Q: Does the smartphone teslameter give the exact value of the field? A: It gives a good estimate but carries a device-specific offset and measurement noise. It is perfectly suited to comparing values and studying laws, less so to a precise absolute measurement.
Q: Why does the value not drop to zero away from any magnet? A: The magnetometer permanently measures the Earth’s field, about 47 µT in western Europe. It is the baseline value on top of which other fields add up.
Q: Can I measure a very intense field with my phone? A: No, beyond the sensor’s range (often a few thousand µT), the measurement saturates. Near a powerful magnet, you must therefore keep your distance to stay within the usable range.
Related concepts
Magnetometer - Earth’s magnetic field - Compass - Magnetic declination - Magnetic inclination