Ballistic method magnetization curve testing is a classical point-by-point DC magnetic measurement technique used to determine the B–H magnetization curve, hysteresis loop, coercivity, remanence, and permeability of soft magnetic materials. Accredited third-party laboratories routinely perform this test in accordance with IEC 60404-4, ASTM A596/A596M, and GB/T 13012 to support material qualification, incoming inspection, and electromagnetic design. This article explains the measurement principle, specimen requirements, test items, procedures, and typical applications of the method.
Technical Overview
The ballistic method (also known as the impulse method) is one of the most established techniques for measuring the DC magnetic properties of magnetically soft materials. In this method, the specimen—most commonly a ring (toroidal) core fitted with a magnetizing winding and a search coil—is magnetized by a stepwise or reversed direct current. According to Faraday's law of electromagnetic induction, the sudden change of magnetic flux induces a voltage pulse in the search coil, and the time integral of this voltage is proportional to the flux change. In the classical implementation, the induced charge produces a ballistic throw of a ballistic galvanometer; in modern implementations, an electronic integrating fluxmeter performs the same integration digitally.
Because the measurement is carried out point by point under quasi-static DC conditions, the ballistic method yields reference-grade values of the initial magnetization curve, the normal (commutation) magnetization curve, the full hysteresis loop, and derived parameters such as coercive field strength Hc, remanence Br, and maximum permeability. These results are largely free from the eddy-current and frequency-dependent effects that complicate AC measurement methods.
The principal standards governing this technique include IEC 60404-4 (methods of measurement of DC magnetic properties of magnetically soft materials), ASTM A596/A596M (DC magnetic properties of materials using the ballistic method and ring specimens), GB/T 13012 (methods of measurement of DC magnetic properties of soft magnetic materials), and the related ASTM A341/A341M. Testing performed by accredited third-party laboratories to these standards is widely accepted for material certification, product qualification, and design verification.
Testing Samples
Ring (Toroidal) Specimens
The ring specimen is the preferred geometry because it forms a naturally closed magnetic circuit, eliminates demagnetizing-field corrections, and allows the circumferential field strength to be calculated directly from the ampere-turns. Solid rings can be machined from cast or wrought alloys such as pure iron, iron-cobalt, and nickel-iron grades, while tape-wound (strip-wound) ring cores are prepared from thin strips of silicon steel, permalloy, amorphous ribbon, or nanocrystalline ribbon. To keep the field distribution acceptably uniform, the radial width should be small compared with the mean diameter; a low ratio of outer to inner diameter is generally recommended. The magnetizing winding and the search coil are distributed uniformly around the circumference of the ring.
Bar, Rod, and Strip Specimens
When a material cannot be prepared as a ring—for example, long bars, rods, or thick sections—permeameter fixtures are used to close the magnetic circuit through laminated yokes and pole pieces. The magnetic field is measured near the specimen surface, and the flux change is captured by a search coil wound directly on the specimen. Permeameter-based measurements are covered as an alternative procedure in IEC 60404-4 and related ASTM test methods, at somewhat reduced accuracy compared with the ring method.
Specimen Preparation and Conditioning
Specimens should be stress-relief annealed where the material specification requires it, since machining or winding stresses can significantly alter permeability and coercivity. The cross-sectional area is determined from dimensional measurements or, for tape-wound cores, from mass and density. Before testing, the specimen must be thoroughly demagnetized, either by a gradually decaying alternating field or by stepwise reversals of decreasing DC field. Tests are normally performed at a controlled ambient temperature, typically 23 ± 5 °C.
Testing Items
Typical measured and derived quantities obtained from ballistic method magnetization curve testing include:
- Initial magnetization curve B(H), starting from the demagnetized state
- Normal (commutation) magnetization curve obtained under cyclic magnetization conditions
- Full DC hysteresis loop for a specified maximum field strength
- Permeability curve μ(H), including initial permeability μi and maximum permeability μm
- Coercive field strength (coercivity) Hc
- Remanence (residual flux density) Br
- Magnetic flux density at specified field strengths, for example B at H = 500, 800, 1600, or 8000 A/m, as required by the material specification
- Hysteresis loss per magnetization cycle Wh, equal to the area of the hysteresis loop ∮H dB, expressed in joules per cubic meter
Testing Methods
Measurement Principle and Circuit
The measuring circuit comprises a stabilized DC power supply, an ammeter, a reversing switch, a rheostat for current adjustment, the magnetizing winding N1, the search coil N2 connected to the ballistic galvanometer or electronic fluxmeter, and a standard mutual inductor used for calibration. The mean circumferential field strength in a ring specimen is calculated as H = N1 · I / lm, where I is the magnetizing current and lm is the mean magnetic path length. When the magnetizing current is suddenly reversed, the flux linkage change in the search coil induces an electromotive force e2(t) = −N2 · dΦ/dt; the integral of this pulse drives an electric charge through the measuring circuit that is proportional to the flux change ΔΦ. Since a full current reversal changes the flux density from +B to −B, the measured flux change gives ΔB = 2B, from which B is obtained directly.
Calibration of the Measuring System
The flux sensitivity of the galvanometer or fluxmeter circuit is calibrated by passing a known current change ΔI through the primary winding of the standard mutual inductor of mutual inductance M, which links a known flux change M · ΔI to the measuring circuit. From the resulting deflection or reading, the flux constant of the system is determined. This calibration establishes metrological traceability and is verified periodically. Corrections for the air flux between the search coil and the specimen may be applied at high field strengths where the air contribution becomes significant.
Test Procedure
- Demagnetize the specimen completely using a decaying AC field or stepwise decreasing reversed DC fields.
- For the initial magnetization curve, switch the magnetizing current on in small increments from the demagnetized state, recording the flux change and cumulative flux density at each field step.
- For the normal magnetization curve, apply repeated current reversals at each step to establish a symmetric cyclic magnetic state, then measure the flux change on reversal, from which B = ΔB / 2 is computed at each field strength H.
- For the hysteresis loop, first cyclically magnetize the specimen to the specified maximum field, then reduce the current stepwise from +Imax through zero to −Imax, recording each incremental flux change; the measured half-loop is mirrored to reconstruct the full symmetric loop.
- Extract Br from the loop intercept at H = 0, determine Hc by interpolation at B = 0, compute permeability values from the curve, and evaluate the loop area for hysteresis loss.
- Report all results together with specimen data, winding parameters, maximum field strength, and the applicable standard.
Practical Considerations and Measurement Uncertainty
Several physical effects must be controlled to ensure accurate results:
- Eddy currents induced by rapid flux changes bias the reading in electrically conducting specimens; the effect is minimized by using slowly ramped current changes and subdivided or thin-gauge specimens.
- Magnetic after-effect (disaccommodation and viscosity) causes readings to drift after each field change; a standardized waiting time is applied before each reading.
- Non-uniform winding distribution and an excessive radial width of the ring degrade the accuracy of the calculated field strength.
- Galvanometer zero drift or fluxmeter offset must be checked and compensated before each reading.
- With proper technique, expanded measurement uncertainties of approximately 1–3 % in flux density and field strength are achievable by experienced laboratories, with larger uncertainty for low-field permeability values.
Applications
Ballistic method magnetization curve testing is applied across the entire lifecycle of soft magnetic materials, from research and development to production quality control:
- Qualification of soft magnetic alloys—pure iron, silicon steel, nickel-iron permalloy grades, iron-cobalt alloys, amorphous and nanocrystalline ribbons—against material standards and purchase specifications
- Design data acquisition for transformers, DC chokes and reactors, relays and contactors, solenoid actuators, electromagnet pole pieces, and magnetic shielding assemblies
- Process optimization of heat treatment, annealing atmosphere, rolling and winding parameters, and alloy composition in materials development
- Incoming material inspection and supplier quality control for manufacturers of magnetic components
- Failure analysis and aging studies, where degradation of permeability or an increase in coercivity indicates material damage or contamination
- Generation of reference DC magnetic data used to verify and cross-check faster routine measurement instruments
FAQ
What is the ballistic method in magnetization curve testing?
The ballistic method is a point-by-point DC magnetic measurement technique in which the flux change in a search coil, caused by a sudden step or reversal of the magnetizing current, is integrated by a ballistic galvanometer or an electronic fluxmeter. By repeating this process at successively higher field strengths, the complete B–H magnetization curve and hysteresis loop of a soft magnetic material are constructed.
Which standards apply to ballistic method magnetization curve testing?
The most commonly applied standards are IEC 60404-4, ASTM A596/A596M, and GB/T 13012, with ASTM A341/A341M covering related permeameter-based procedures. Reports issued by accredited third-party laboratories with reference to these standards are widely accepted for certification and regulatory purposes.
How does the ballistic method differ from AC B–H testers or a vibrating sample magnetometer?
The ballistic method measures true DC properties under quasi-static conditions, free from eddy-current effects, whereas AC B–H testers measure frequency-dependent properties of sheet materials. A vibrating sample magnetometer measures open-circuit magnetization of small samples and is better suited to hard magnetic materials; the ballistic ring method provides higher accuracy for the DC properties of soft magnetic materials.
What specimen should be submitted for testing?
A ring specimen is preferred. Typical acceptable dimensions are an outer diameter of roughly 40–60 mm with a proportionally small radial width, though laboratories can machine rings from larger stock or supply the required magnetizing and search windings on customer-furnished cores. The specimen mass and dimensions should be provided so the cross-sectional area can be determined accurately.
What accuracy can be expected?
With a well-prepared ring specimen and calibrated instrumentation, expanded uncertainties of approximately 1–3 % for flux density and field strength are typically achievable. Permeability at very low fields and parameters of materials with strong time-dependent behavior carry somewhat larger uncertainty.
Can permanent magnet materials be tested with this method?
No. The magnetizing fields available in a ballistic ring setup are insufficient to saturate hard magnetic materials. Permanent magnets are characterized with pulsed-field hysteresisgraphs or vibrating sample magnetometers in accordance with standards such as IEC 60404-5.