Initial permeability testing by the ballistic (shock) method determines the true direct-current permeability of soft magnetic materials at very low field strengths by integrating the voltage induced in a search coil during a sudden change of the magnetizing current. This article explains the measurement principle, applicable specimens, test items, procedures, calculations, and relevant international standards for this classical technique as performed by accredited third-party laboratories and research facilities.
Technical Overview
Initial permeability (µi) is one of the most important figures of merit for soft magnetic materials. It is defined as the limit of the ratio of flux density B to the applied field strength H, referred to the permeability of free space µ0, as H approaches zero, starting from a completely demagnetized state. Because µi reflects the ease with which domain walls move at vanishing excitation, it directly governs the low-signal inductance, sensitivity, and noise performance of transformers, inductors, current sensors, and magnetic shielding components.
The ballistic method, historically known in Chinese-language literature as the shock method and classically implemented with a ballistic galvanometer, determines flux density by measuring the total voltage impulse induced in a search coil when the magnetizing current in a primary winding is suddenly changed or reversed. According to Faraday's law, the time integral of the induced voltage equals the product of the number of secondary turns, the cross-sectional area of the specimen, and the change in flux density. Modern implementations replace the galvanometer with high-resolution electronic or digital fluxmeters that perform voltage integration with automatic drift compensation, greatly improving convenience and repeatability while preserving the underlying principle.
Compared with the widely used alternating-current inductance bridge method, the ballistic method provides a genuine quasi-static (direct-current) value of permeability that is essentially free of eddy-current and frequency-dependent loss contributions. This makes it particularly valuable for electrically conductive alloys such as nickel-iron permalloys, silicon-iron, and iron-cobalt alloys, as well as for soft magnetic composites and nanocrystalline ribbons, whenever a standard, specification, or research objective requires the true DC initial permeability rather than an effective AC value.
A critical aspect of the technique is magnetic conditioning. Before any low-field measurement, the specimen must be thoroughly demagnetized, typically by applying a slowly decaying alternating field, so that the measurement starts from a reproducible, history-free magnetic state. Immediately after demagnetization, permeability may drift with time due to disaccommodation, so the waiting time between demagnetization and measurement must be controlled and reported in accordance with the governing standard.
Testing Samples
Ring (Toroidal) Specimens
The closed magnetic circuit of a ring specimen is the preferred geometry because it eliminates demagnetizing effects and allows the internal field strength to be calculated directly from the ampere-turns and the mean magnetic path length. Typical samples include sintered soft ferrite rings, pressed and annealed powder rings, machined rings cut from bulk alloy billets, and stacked laminations punched or wound into ring form. The laboratory usually winds the primary (magnetizing) and secondary (measuring) coils directly onto the specimen after insulating it.
Tape-Wound Cores
Amorphous and nanocrystalline ribbons, as well as thin-gauge permalloy strips, are commonly supplied as tape-wound toroidal cores that have already been stress-relief annealed. These cores are well suited to ballistic testing provided the winding is performed without imposing mechanical stress, since even slight strain can measurably reduce the initial permeability of these materials.
Bars, Strips, and Bulk Pieces
When a closed specimen cannot be produced, bars, rods, and strips may be tested in a permeameter adapted to the impulse principle, in which the field is applied by a yoke system and the flux is sensed by a surrounding coil. Open-circuit configurations require additional correction for the demagnetizing factor and generally yield less accurate low-field results, so ring specimens remain strongly recommended for initial permeability determination.
Specimen Preparation Requirements
- Known outer diameter, inner diameter, and height, or known mass and material density, so that the effective cross-sectional area and mean magnetic path length can be calculated.
- A ratio of outer to inner diameter preferably close to unity (commonly no greater than about 1.4, depending on the standard) so that the field is approximately uniform along the path.
- Stress-relief annealing performed before testing whenever the machining, cutting, or winding process may have introduced internal stress.
- Clean, insulated surfaces between windings and specimen to prevent shorted turns.
- Documented thermal stabilization at the specified test temperature before measurement.
Testing Items
Although the ballistic method is most closely associated with initial permeability, a single test session on a properly instrumented ring specimen can deliver a complete set of direct-current magnetic characteristics, including:
- Initial permeability (µi) at specified low field strength or by extrapolation of the B–H curve to zero field.
- Normal magnetization curve (B versus H) obtained point by point with stepwise current increases.
- Permeability as a function of field strength, including maximum permeability (µmax).
- Full direct-current hysteresis loop after saturation.
- Coercive field strength (Hc) and remanent flux density (Br).
- Saturation flux density (Bs) at the maximum available field.
- Disaccommodation-related behavior, when the time dependence of permeability after magnetic conditioning is of interest.
- Temperature dependence of initial permeability, when a temperature-controlled enclosure is used.
Testing Methods
Instrumentation and Setup
A typical arrangement consists of a stable direct-current source, a precision ammeter or calibrated shunt, a low-resistance reversing and stepping switch, the ring specimen with its primary and secondary windings, and a digital fluxmeter or ballistic galvanometer with a known calibration constant. A temperature chamber may surround the specimen when temperature-dependent measurements are required. All instruments should be traceably calibrated, and the fluxmeter should be zeroed with drift compensation active immediately before each current step.
Measurement Procedure
- Record specimen dimensions, mass, and material density; calculate the mean magnetic path length lm, taken as π multiplied by the average of the outer and inner diameters, and the effective cross-sectional area Ae, which may be obtained from mass divided by the product of density and path length.
- Wind and document the primary turns N1 and secondary turns N2, insulating the specimen beforehand.
- Allow the specimen to stabilize thermally at the specified temperature, commonly 23 ± 5 °C unless otherwise agreed.
- Demagnetize the specimen with a gradually decaying alternating field of suitable amplitude, or by an equivalent stepwise DC reduction to zero.
- Observe the standardized waiting period after demagnetization to control disaccommodation effects.
- Apply the smallest current step, record the integrated flux reading and the corresponding current, and compute the incremental flux density and field strength.
- Repeat with progressively larger steps to trace the initial region and, where required, the complete magnetization curve up to saturation.
- For the hysteresis loop, saturate the specimen, then reverse or reduce the current in controlled steps to determine Br and Hc.
- Extrapolate the low-field slope of the B–H characteristic to zero field to obtain the initial permeability, and report all test conditions.
Data Processing and Calculation
The field strength generated by each current step is calculated as H = N1 × I / lm, where I is the magnetizing current. The change in flux density follows from the fluxmeter reading according to ΔB = (1 / (N2 × Ae)) × the integral of the induced secondary voltage. The relative permeability for each step is then ΔB divided by the product of µ0 and ΔH, and the initial permeability is reported as the limiting value as the field increment approaches zero, or at the low field strength defined by the applicable standard. In classical ballistic galvanometer operation, ΔB is proportional to the observed deflection, with the instrument constant determined by calibration using a mutual inductor. Uncertainty is evaluated from the contributions of dimensional and mass measurements, current and fluxmeter calibration, winding count, and temperature control, with expanded uncertainties of a few percent being typical for well-prepared ring specimens.
Reference Standards
- IEC 60404-4, covering methods of measurement of direct-current magnetic properties of magnetically soft materials using permeameter and ballistic fluxmetric techniques.
- ASTM A596/A596M, covering direct-current magnetic properties of materials using ring specimens and the ballistic method.
- ASTM A341/A341M, covering direct-current permeability measurement with permeameter and fluxmeter instrumentation.
- GB/T 3658, the Chinese national standard for the measurement of direct-current magnetic properties of soft magnetic materials by the ballistic method.
- IEC 60404-6 and the IEC 62044 series, which define complementary ring-specimen and soft magnetic core measurement procedures useful for comparing ballistic results with alternating-current methods.
Applications
Ballistic initial permeability testing supports product development, incoming inspection, qualification, and failure analysis across every industry that relies on precision soft magnetic components. Typical application areas include:
- High-permeability nickel-iron and iron-cobalt alloys for magnetic shielding, precision transformers, and low-noise sensors.
- Sintered MnZn and NiZn soft ferrites for signal transformers, filter inductors, and broadband devices.
- Amorphous and nanocrystalline tape-wound cores for common-mode chokes, current transformers, and high-efficiency power magnetics.
- Soft magnetic powder and composite cores for DC-biased inductors in power electronics.
- Electrical steel evaluation for research comparisons between DC and power-frequency characteristics.
- Aerospace, defense, and energy-sector programs in which DC permeability values are contractually specified.
- University and industrial research on new alloys, annealing recipes, and stress or radiation effects on soft magnetic behavior.
FAQ
Q: How does the ballistic method differ from the LCR bridge method for initial permeability?
A: An LCR bridge measures the effective AC permeability at a chosen frequency and flux level, which includes frequency-dependent eddy-current and loss effects. The ballistic method measures the true quasi-static DC value, which is independent of frequency and is required whenever a specification defines initial permeability on a direct-current basis.
Q: Why is demagnetization so important in this test?
A: Demagnetization removes the magnetic history of the specimen and establishes a reproducible starting point at H = 0. Without it, the measured low-field permeability depends on prior magnetization and the results are not comparable. Because permeability can drift after demagnetization, the elapsed time before measurement must also be standardized.
Q: What sample should be submitted for the most accurate result?
A: A closed ring specimen with a nearly uniform path, known geometry or mass and density, and no machining stress is ideal. Open bars or strips can be tested in a permeameter, but corrections for the demagnetizing factor reduce accuracy at low fields.
Q: What accuracy can be expected?
A: For well-prepared ring specimens measured with calibrated instruments in a controlled environment, expanded uncertainties on the order of three to five percent are typical, with dimensional and mass determination usually being the dominant contributors.
Q: Can the test be performed at other temperatures?
A: Yes. Initial permeability is temperature sensitive, particularly in ferrites near their permeability minima, so measurements at multiple temperatures are common for automotive, aerospace, and industrial qualification programs.
Q: How long does a typical test take?
A: A single-temperature measurement on a ready specimen, including winding, demagnetization, and reporting, is usually completed within a few working days, while multi-temperature or full hysteresis characterizations require additional scheduling.