Real‑world examples of using a galvanometer to measure magnetic flux

Lab-tested examples of using a galvanometer to measure magnetic flux

The best way to understand magnetic flux is to watch a needle jump. That’s why physics instructors love examples of using a galvanometer to measure magnetic flux: the instrument turns an invisible field into a visible deflection.

In all of the examples below, the same basic idea repeats:

• A coil of wire experiences a change in magnetic flux.
• An induced emf drives a tiny current through the circuit.
• The galvanometer responds with a deflection proportional to that current.

Faraday’s law in its standard form is

\(\mathcal{E} = -\dfrac{d\Phi_B}{dt}\)

where \(\mathcal{E}\) is the induced emf and \(\Phi_B\) is the magnetic flux through the coil. The galvanometer doesn’t measure flux directly; it measures the current caused by that changing flux. But with a known coil resistance and geometry, you can back out the flux change from the observed deflection.

Classic classroom example of flux change: bar magnet and coil

One of the simplest examples of using a galvanometer to measure magnetic flux appears in almost every introductory physics lab.

You wind a coil of, say, 500–1000 turns around a cardboard tube. The coil is connected in series with a sensitive moving-coil galvanometer. Then you take a bar magnet and move it toward and into the coil.

Here’s what you see:

• As the north pole of the magnet approaches the coil, the galvanometer needle deflects in one direction.
• When you hold the magnet still, the needle returns to zero. The flux is nonzero, but not changing.
• When you pull the magnet out, the needle deflects in the opposite direction.

By repeating the motion at different speeds, you get a family of real examples where faster motion produces larger peak deflections. That’s a direct, visual way to confirm that \(d\Phi_B/dt\), not just \(\Phi_B\), determines the induced current.

In more advanced versions of this experiment, students measure the maximum deflection for different speeds and estimate the rate of flux change, using the galvanometer’s calibration and the coil resistance.

Rotating coil in a uniform magnetic field: generator-style examples

A slightly more sophisticated example of using a galvanometer to measure magnetic flux uses a rotating coil between the poles of a strong electromagnet. This is essentially a stripped-down AC generator used purely as a teaching tool.

The setup looks like this in words:

• A rectangular coil with \(N\) turns rotates at a controllable angular speed \(\omega\) in a nearly uniform magnetic field \(B\).
• The coil is connected through slip rings and brushes to a galvanometer.

As the coil rotates, the effective area exposed to the field changes as \(A\cos \theta\). The magnetic flux becomes

\(\Phi_B = N B A \cos(\omega t)\)

Differentiating gives a sinusoidal induced emf and a sinusoidal current that the galvanometer can track as a periodic deflection. In low-speed demos, students can see the needle swing back and forth in time with the rotation.

This is one of the best examples of using a galvanometer to measure magnetic flux over time because you can:

• Change the rotation rate and see the deflection amplitude grow.
• Reverse the rotation direction and watch the phase of the deflection flip.
• Insert different core materials (air, iron, ferrite) and watch how the peak deflection changes with effective \(B\).

These examples include direct comparisons that highlight how material properties and motion affect flux.

Measuring flux linkage in transformer cores

If you want a more engineering-flavored example of using a galvanometer to measure magnetic flux, transformer-core experiments are ideal.

In a typical undergraduate lab, you have:

• A laminated iron core.
• A primary winding driven by a low-voltage AC or pulsed DC source.
• A secondary “search coil” with relatively few turns connected to a ballistic galvanometer.

By applying a known current pulse in the primary, you change the magnetization of the core and therefore the magnetic flux through the secondary. The galvanometer’s transient deflection is proportional to the total change in flux linkage \(N\Delta \Phi_B\) through the secondary coil.

Students can:

• Vary the primary current and see how the galvanometer deflection scales.
• Compare air-core versus iron-core behavior.
• Map out the B–H curve of the core material by combining current measurements with galvanometer readings.

This gives real examples of using a galvanometer to measure magnetic flux in a context that looks a lot like power engineering. Many modern lab manuals from universities such as MIT and other institutions (for instance, physics lab resources linked via MIT OpenCourseWare or Harvard’s physics labs) still describe versions of this experiment in 2024.

Earth’s magnetic field: search coil and galvanometer

For students who like “big picture” applications, measuring changes in Earth’s magnetic field offers one of the more eye-opening examples of using a galvanometer to measure magnetic flux.

You don’t measure the static geomagnetic field directly with a galvanometer—that’s better done with magnetometers—but you can measure changes in flux using a search coil:

• A multi-turn coil is oriented along a known direction (for example, aligned with the local horizontal component of Earth’s field).
• The coil is suddenly rotated through 90 degrees.
• The coil is connected to a ballistic galvanometer, which records a single, brief deflection proportional to the total change in flux.

From this, students estimate Earth’s field magnitude using geometry and the coil’s area and turn count. Updated geomagnetic field values (for comparison) are available from agencies like the U.S. National Centers for Environmental Information and the British Geological Survey, which publish current field models and maps.

In 2024, as satellite and ground-based magnetometer networks continue to refine our understanding of space weather, this old-school coil-and-galvanometer approach still serves as a tangible example of how changing magnetic flux is detected in principle.

Eddy currents and shielding: practical lab examples

Another set of examples of using a galvanometer to measure magnetic flux focuses on eddy currents and magnetic shielding.

Here’s a standard lab narrative:

• A coil connected to a galvanometer is placed near a region where a magnetic field is pulsed—often using a neighboring drive coil.
• First, the coil is left in free space, and the galvanometer deflection is recorded for each pulse.
• Next, a conductive plate (aluminum or copper) is inserted between the drive coil and the sensing coil.

Students observe that the galvanometer deflection decreases when the conductive plate is present. The eddy currents in the plate create opposing magnetic fields that partially cancel the changing flux reaching the sensing coil.

These real examples show how induced currents can be both a nuisance (energy loss in transformers) and a tool (magnetic braking, shielding). In modern technology, related principles appear in induction cooktops, MRI gradient coils, and magnetic levitation systems, all of which rely on controlling how flux changes in conductive or ferromagnetic materials.

Flux in superconducting loops: bridging to modern research

In advanced university labs and research settings, you occasionally see more exotic examples of using a galvanometer to measure magnetic flux in the context of superconductivity.

While modern experiments often use digital electronics and SQUIDs (superconducting quantum interference devices), analog galvanometers still appear in teaching labs as a bridge between classic and modern methods:

• A superconducting loop is cooled below its critical temperature.
• A nearby coil produces a controlled, time-varying magnetic field.
• A secondary pickup coil, linked to the superconducting loop, is wired to a sensitive galvanometer.

When the applied field changes, the superconducting loop resists changes in its total flux (flux quantization), and induced currents adjust to keep the flux nearly constant. The pickup coil detects these adjustments as induced currents, which the galvanometer registers.

These are not everyday classroom demos, but they are among the best examples for showing how the simple idea of “galvanometer + changing flux” scales all the way up to cutting-edge quantum devices. For background on superconductivity and flux quantization, the U.S. National Institute of Standards and Technology (NIST) offers accessible explanations and technical references.

Calibrating coil areas and turn counts with a galvanometer

Sometimes the goal isn’t to study the field; it’s to calibrate the hardware. That leads to another family of examples of using a galvanometer to measure magnetic flux where the coil itself is the unknown.

Imagine you have a coil of unknown effective area or turn count. You place it inside a solenoid whose magnetic field you can calculate from \(B = \mu_0 n I\), where \(n\) is turns per unit length and \(I\) is the current. You then:

• Pulse the solenoid current from zero to a known value.
• Record the galvanometer deflection from the coil.

Since \(\Delta \Phi_B = N A \Delta B\), and you know \(\Delta B\) from the solenoid current, you can solve for the product \(N A\). Repeating this with several coils gives a series of real examples of using a galvanometer to measure magnetic flux in order to characterize the coils themselves.

This technique is still used in metrology labs and advanced teaching labs as a low-cost way to cross-check coil specifications without relying solely on manufacturer data.

How these examples connect to modern magnetic sensing

By 2024–2025, most industrial and research-grade magnetic measurements use digital sensors—Hall probes, fluxgate magnetometers, optically pumped magnetometers, and SQUIDs. Agencies like NASA and NOAA rely on such sensors for space weather monitoring and Earth field mapping, as described in resources linked via NASA’s Heliophysics pages and NOAA’s geomagnetic services.

So why keep teaching examples of using a galvanometer to measure magnetic flux?

Because they provide:

• A clear, visual connection between field changes and induced current.
• A way to see Faraday’s law before it disappears into black-box electronics.
• A direct, analog sense of polarity and direction—students can literally see Lenz’s law in the direction of the needle swing.

Modern labs often pair these classic experiments with digital data acquisition: the same coil output is fed into an analog-to-digital converter while the galvanometer gives a live analog display. This hybrid approach lets students compare the traditional needle deflection with actual current waveforms on a screen, turning old-school examples of using a galvanometer into a stepping stone toward contemporary instrumentation.

FAQ: common questions about galvanometers and magnetic flux

What are some simple examples of using a galvanometer to measure magnetic flux in a school lab?

Common classroom examples of using a galvanometer to measure magnetic flux include moving a bar magnet in and out of a coil, rotating a coil in a magnetic field, and using a search coil to detect changes in a solenoid’s field. All of these rely on changing flux to induce a current that the galvanometer can display.

Can a galvanometer measure constant magnetic flux directly?

No. A galvanometer in these experiments responds to current, which is driven by a changing flux. If the magnetic field is constant and the coil isn’t moving or changing shape, the flux is steady and the galvanometer reads zero, even if the actual flux is large.

What is an example of using a galvanometer to study Earth’s magnetic field?

A classic example of this is the search-coil method: you orient a coil along Earth’s field, suddenly rotate it, and measure the induced pulse with a ballistic galvanometer. From the deflection, you estimate the change in flux and back-calculate Earth’s field strength.

Why use a ballistic galvanometer instead of a regular one in some experiments?

A ballistic galvanometer is designed to respond to the total charge that passes through it during a brief pulse, rather than the steady current. That makes it ideal for experiments where the flux change happens quickly, such as snapping a coil from one orientation to another or pulsing a solenoid.

Are these classic galvanometer experiments still relevant with modern sensors?

Yes. While modern magnetometers are far more sensitive and convenient, these real examples with galvanometers give students a direct, visual understanding of Faraday’s law and Lenz’s law. Once that intuition is in place, it’s much easier to appreciate how advanced magnetic sensors work under the hood.