The History of ET

Chapter 1: IntroductionPurpose of This ChapterThis chapter introduces the scope and fundamental purpose of Electromagnetic Testing (ET), a nondestructive test method, with a particular focus on the Eddy Current Testing (ECT) technique. It also presents a brief history of how the field evolved from basic magnetic discoveries to the early development of impedance-based instrumentation.This teaching version helps the reader place Libbys work in historical and technical context not to improve todays testing, but to understand how foundational ECT theory was developed in the 20th century.1.1 What Is Electromagnetic Nondestructive Testing?Electromagnetic NDT is any inspection method that uses time-varying magnetic or electric fields to detect, measure, or evaluate a material without damaging it.The Eddy Current Testing (ECT), Remote Field Testing (RFT) and Alternating Current Field Measurement (ACFM) techniques all fall under the umbrella of the Electromagnetic Testing (ET) method. ECT relies on electromagnetic induction to generate circulating currents in a conductive test object. These currents, known as eddy currents, are influenced by the materials electrical and magnetic properties, as well as by any discontinuities such as cracks, corrosion, or material loss.ECT is often used to inspect:- Tubing (in power plants, condensers, or heat exchangers)- Aircraft structures- Conductive coatings- Weld zonesThe goal is to measure changes in the coil's impedance or voltage output that result from test object variations such as cracks, thickness changes, or material properties and to interpret those signals with high confidence.1.2 The Basic Workflow of an Eddy Current TestEven in Libbys time, ECT followed the same basic functional steps that are used today:1. Excitation: A sinusoidal (AC) current is applied to a test coil.2. Field Coupling: This current produces a primary magnetic field, which couples with the test object.3. Eddy Current Generation: The time-varying field induces eddy currents in the material.4. Field Interaction: The eddy currents create a secondary magnetic field that modifies the impedance of the coil.5. Signal Analysis: The modified coil voltage or current is measured and analyzed.6. Interpretation: The resulting signal is used to infer properties or detect flaws in the material.1.3 Historical Development of Electromagnetic TestingUnderstanding Libbys book requires some appreciation for the scientific lineage that preceded him. Much of Libbys thinking was shaped by:- James Clerk Maxwell (1870s): Unified the understanding of electric and magnetic fields via Maxwells Equations.- Michael Faraday (1831): Discovered electromagnetic induction, showing that changing magnetic fields can induce currents.- Dr. Frederick Frster (1930s1950s): Developed early impedance plane display systems and laid the foundation for signal-based flaw detection.- Richard Hochschild: Contributed to understanding coil behavior and signal interpretation, influencing Libby directly.Libby was working at the Hanford Laboratory in Richland, WA, during the 1950s and 60s, trying to convert these theoretical breakthroughs into usable instrumentation often constrained by early analog electronics, CRT displays, and limited signal processing.Before Hugo Libby ever rotated a signal vector or displayed a flaw on a CRT, Dr. Friedrich Frster had already mapped out the impedance plane.Working in Germany in the 1930s1950s, Frster built some of the first practical eddy current test systems, including:Opposing-coil bridge probesAnalog phase interpretation methodsCRT-based display systems for flaw characterizationHis legacy wasnt limited to invention it was foundation. Libbys later work on signal discrimination, flaw vector analysis, and analog matrix filtering can be traced back to Frsters original visual and physical models.In 1948, nondestructive testing was still in its formative years. The Second World War had dramatically accelerated demand for reliable inspection methods, and the U.S. industrial base was exploring the earliest forms of ultrasonic, radiographic, and electromagnetic inspection.This period marked a turning point. While magnetic particle and radiographic testing dominated heavy industry, eddy current methods were gaining attentionbut were still viewed as relatively novel, particularly in the United States.The Nondestructive Testing Methods manual published that year captured a snapshot of the field:Emphasis on basic flaw detection rather than quantitative analysisLimited discussion of electromagnetic theoryreflecting the gap Libby would later work to closeA general awareness of induction methods, but little practical infrastructure for complex coil designs or multifrequency analysisLibbys later work in the 1950s and 60s would build directly on this foundationbringing mathematical rigor, signal phase interpretation, and frequency-domain strategies to a field that was still largely empirical in 1948.In 1955, Dr. Friedrich Frster published a landmark paper introducing the DC magnetic field and difference field meter, known today as the Frster Probe. This paper formalized the use of dual-coil systems to measure not just absolute field strength, but also minute magnetic field gradientsa principle central to modern eddy current and magnetic flux leakage testing.Frster's innovation was elegant: by winding two magnetization coils in opposite directions and analyzing the second harmonic component of the induced signal, he could isolate DC field strength with extraordinary sensitivityas low as 10 Oersted.Even more important was his invention of the difference field probe:By simply turning one of the two probe coils into the opposite direction, the system becomes insensitive to uniform DC fields and instead indicates the gradient between themallowing field difference measurements independent of ambient field interference.This design made possible a broad range of novel NDT techniques:Residual magnetism mappingLocal coercivity measurementPointwise magnetic hardness testingFerromagnetic inclusion detection in nonmagnetic matricesTrue wall thickness measurement (ferrous and non-ferrous)Flaw detection via leakage flux recording (precursor to today's MFL)Frsters method would go on to influence generations of probe-based detection systemsnot just in magnetic inspection, but in the way eddy current systems later evolved to focus on localized magnetic interactions and field mapping as signal sources.Optional Visual Enhancements:Diagrams from Figures 2, 4, or 6 could visually anchor this section in your book.Create a comparison matrix showing:While Hugo Libby and others were developing phase-based eddy current testing systems in the 1950s, the magnetic testing world was undergoing its own technological renaissance.In a 1950 industry article, V.L. Spoley of Magnetic Analysis Corporation described how the earliest magnetic testing devicessimple circuits with galvanometerswere quickly evolving into complex electronic systems using 60+ vacuum tubes, oscilloscopes, and phase-shifting networks.This quote from the article captures the spirit of postwar innovation:Twenty years ago the steel industry was content to have equipment which was capable of detecting cracks in plain carbon steel bars... Todays equipment must inspect wire of .020-inch diameter and detect flaws no deeper than .0015 inchesall at production speeds exceeding 200 feet per minute.Whats striking is the shared set of challenges both magnetic and eddy current methods faced:Detecting smaller, subtler flawsOperating at higher inspection speedsAccommodating greater material diversityMoving from visual indicators to impedance-based or phase-based decisionsLibbys work helped advance eddy current testing just as the Multi-Method magnetic systems were maturing. Both disciplines drew on parallel breakthroughs in electronics, signal processing, and materials science to transition from bench-top experiments to essential production tools.In 1955, Glenn O. McClurg formalized the mathematics behind coil magnetization, introducing a now-classic concepteffective permeability (_eff)which accounts for how part shape affects magnetic flux density in the test object.He showed that the internal field of a part placed in a coil is weakened by self-demagnetization, a function of its shape and how it interacts with the magnetic field:Magnetic poles appear near the surfaces at which the field enters and leaves the part... producing a field in the part which opposes the applied field.Using this insight, McClurg derived the effective permeability as:eff=BH0=11+N4(1)\mu_{\text{eff}} = \frac{B}{H_0} = \frac{1}{1 + \frac{N}{4\pi}(\mu - 1)}eff=H0B=1+4N(1)1or approximately:eff6(ld)5\mu_{\text{eff}} \approx 6\left(\frac{l}{d}\right) - 5eff6(dl)5where l/dl/dl/d is the length-to-diameter ratio of a cylindrical part, and N/4N/4\piN/4 is the demagnetizing factor.This directly supports Libbys own efforts to quantify coil loading and magnetic interaction, albeit from a magnetic particle inspection perspective.McClurg then translated this into a thumb-rule for required ampere-turns to achieve the necessary flux density for inspection:Ampere-turns=45,000l/d\text{Ampere-turns} = \frac{45,000}{\sqrt{l/d}}Ampere-turns=l/d45,000assuming a target flux density of 70,000 lines/inproven effective for flaw detection.The conceptual leap was huge: by acknowledging that geometry modifies field behavior, McClurg created a bridge between theoretical electromagnetics and practical magnetization coil designa principle central to eddy current coil loading as well.In the early 1950s, while pioneers like Hugo Libby were refining eddy current theory and applying multifrequency signal analysis, other innovators were exploring entirely different physical principles to solve similar problemssuch as metal sorting and material differentiation.One example was the Metalsorter and Thermosorter, instruments developed and promoted by Antony Doscheck. These devices didnt use electromagnetic induction at all. Instead, they leveraged the triboelectric effect (voltage generated by friction between dissimilar metals) and the thermoelectric effect (voltage from temperature gradients across different metals) to identify alloy types and heat-treatment variations.Doscheck wrote:The polarity discriminating ability of the triboelectric test is its most important single feature... Steels, and other heat-treatable alloys, show differences in triboelectric properties which vary as the type of heat treatment.These methods were especially valuable for non-destructive alloy sorting in fabrication shops and warehouseswhere electrical conductivity and frictional interactions could be rapidly assessed without cutting, heating, or scrapping parts.However, they were ultimately limited by:Surface condition sensitivityLack of penetration (surface-only properties)Inability to detect subsurface flaws or phase lagsIn contrast, eddy current testing (particularly as advanced by Libby and Frster) evolved to become more quantitative, multi-parameter, and deeply tunablecapable of flaw characterization, phase discrimination, and material property quantification in production-critical applications.Still, these early tools like the Metalsorter remind us that mid-century NDT was a wide-open frontier, with many competing technologies vying to answer the same industrial questionseach with its own strengths and limits.While Hugo Libby was refining signal-phase interpretation and depth discrimination techniques in the lab, companies like General Electric were already deploying impedance-based eddy current testers on the factory floor.In 1952, G.E. introduced the Metals Comparator, a portable, multi-frequency device capable of inspecting and classifying ferrous and nonferrous metals using eddy current impedance changes. It offered frequencies from 50 Hz to 10,000 Hz, with test coils and handheld test heads suited for various applications.The comparator provides a fast non-destructive test of the quality of ferrous and nonferrous parts. Differences in chemical and physical properties are indicated by changes in the test units impedance.The G.E. Metals Comparator could:Sort mixed metals by resistivity or permeabilityDetect hardness differences and case depthIndicate plating thicknessSupport quality control and even fault detection in some materialsIt worked by balancing a test coil or head using a reference sample. Any variation in a new parts electrical or magnetic properties would deflect the meter reading, identifying alloys, treatments, or defects. Though still analog, this system reflected Libbys core principlesespecially the importance of impedance as a proxy for material condition.These early field instruments marked a transition: from purely academic waveforms and phase angles to industrial-scale usability. Libbys work gave meaning to the signals, while tools like the Metals Comparator delivered speed, repeatability, and standardization.In 1952, W.E. Thomas of Magnalux Corporation delivered a bold message to the NDT community:No process is employed by intelligent industrial management unless it does more than 'pay its way'except when forced by outside pressure.This marked a shift in how NDT professionals needed to thinknot just about detecting flaws or producing pretty scope signals, but about proving the financial value of their work.Thomas outlined scenarios where nondestructive testing could dramatically cut costs, such as:Screening used engine blocks before rebuilding to avoid $60/day in lossesPre-production sorting to prevent processing scrapIn-process checks to catch bad dies or overheated forgings earlyHe even presented a now-classic chart (Figure 3 in the original) comparing cost of inspection to cost of defects caught too late, helping engineers calculate the break-even point where NDT becomes economically justified.This kind of business logic paved the way for broader acceptance of tools like eddy current comparators and multifrequency instruments. While Libby was busy optimizing phase lag to separate flaws from geometry, pioneers like Thomas were showing management how this new signal intelligence translated to profit.Together, these two perspectivestechnical depth and economic clarityhelped mainstream eddy current testing as not just a scientific marvel, but a cost-effective production tool.In 1953, Magnaflux introduced the FM-100 Conductivity Meter, marking one of the first commercially available instruments that could instantly display %IACS conductivity from a handheld probe. This innovation brought quantitative materials analysis into the hands of shop-floor inspectors.The operator places the hand probe on a part and reads the dial. A relatively flat area of only about inch in diameter is required.Applications included:Sorting mixed nonferrous alloys (e.g., copper, brass, aluminum)Verifying heat treatment of aluminum based on conductivity changes due to temperingAssessing melt purity in nonferrous alloy productionWhile Libbys early work focused on multifrequency response and flaw detection using impedance and phase shifts, instruments like the FM-100 represented the practical, simplified face of electromagnetic materials testingdesigned not for deep flaw sizing, but for rapid classification and quality control.This dichotomyLibbys diagnostic depth vs. the comparators simplicitywould continue to shape the development of ECT instruments for decades to come.By the early 1950s, eddy current testing had made the leap from laboratory curiosity to widely adopted industrial tool, thanks in large part to companies like Magnaflux Corporation, which commercialized Dr. Frsters concepts into working instruments such as the Magnatest FM-100.In a 1953 presentation to ASNT, William A. Cannon Jr. described how the FM-100 allowed for direct conductivity readings from nonferrous parts with nothing more than a handheld probe and a calibrated dial:The detector induces eddy currents into the test part. The impedance of the detector will vary depending on the conductivity of the material the bridge is balanced by adjusting the main dial; the conductivity may be read directly.The report described instruments for:Measuring %IACS conductivity from 8% to 103%Detecting cracks and seams in wire using cathode ray tubesMeasuring plating and coating thickness (e.g., chromium on piston rings, ceramic on aircraft parts)Sorting small parts by hardness, alloy, or size at 810k parts/hourThis moment in history marked a convergence:Frster provided the theoryLibby built on it with signal modeling and multifrequency conceptsManufacturers like Magnaflux created usable, high-speed instruments that integrated these principlesIt was the industrial maturity of Libbys visiontechnology that not only worked but could survive in the hands of non-scientists, running 3 shifts a day.While Hugo Libby was advancing multifrequency eddy current flaw detection, physicists like C.H. Hastings and G.A. Darcy at the Watertown Arsenal were refining search coil techniques for use in magnetic inspection of gun barrels.Presented at the same 1952 ASNT conference where Libby and others shared their work, Hastings and Darcy outlined two foundational magnetic inspection methods:Induction method based on voltage induced in a moving coil over a magnetized specimenA.C. Bridge method using impedance changes in a probe to detect leakage fields from cracksThey emphasized a philosophy that echoes Libbys mindset:One cannot get more intelligence from an inspection system than the search coil or other input device can detect.Their work on the Magnetic Recording Boroscopea precursor to today's mapping and digitized inspection toolsproduced facsimile images of bore defects using a mechanically scanned search coil and synchronized recorder. It even outperformed visual and black-light inspections by:Detecting fine quench cracks missed by opticsReducing false calls with improved signal interpretationStandardizing operator performanceThis overlap of ideassmart signal processing, automation, and machine-assisted interpretationshows that whether you used eddy currents or magnetic leakage fields, the goal was the same: Turn invisible flaws into clear, reliable signals.Libbys frequency-phase model and Watertowns mechanical-electronic recorder may have taken different roads, but both aimed at making inspection repeatable, sensitive, and intelligent.In 1953, physicist Gerold H. Tenney of Los Alamos Scientific Laboratory delivered a keynote that read more like a manifesto. It reflected a field that was growing rapidlybut without enough trained people, without enough academic support, and without a unified professional identity.His words remain hauntingly relevant today:There is still much nondestructive testing equipment on the shelves, in the cellars, and warehouses of industrial installations... not being used at all, or used in such a way that the results simply do not compare with the theoretically obtainable quality.Tenney made several key points that mirrored what Hugo Libby would try to solve through his technical contributions:Universities were not teaching NDT. In fact, by 1948, not a single college in the U.S. offered a serious course in NDT methods.Industry lacked standardized training pipelines, relying instead on self-teaching, vendor courses, and peer-to-peer knowledge transfer.The NDT workforce was trapped in a skills bottleneck, unable to meet growing postwar demand for safety, quality, and productivity.Professional societies, such as what became ASNT, were critical to forming a sense of discipline, community, and standards.Tenney even contrasted two job ads:One for a metallurgisteasily filled by college graduates.Another for an X-ray NDT technicianimpossible to fill without poaching someone already trained on the job.This address gives voice to the hidden crisis that Libbys work helped resolve: the lack of interpretable signal intelligence, training frameworks, and technical rigor in a field that was becoming indispensable to American industry.1.4 Why This Matters (Even If It Doesnt Help Your Signal Quality Today)If youre preparing for a Level III exam, its tempting to skip the historical theory. After all, todays instruments handle mixing, phase discrimination, and data analysis automatically. But Libbys work tells the story of:- Why the impedance plane became so central to ECT- How signal modulation by the test object is foundational- What it took to understand and design tests with multiple interacting material variables (a precursor to multifrequency testing)This isnt about making your current test better its about understanding how we got here, and being able to think through problems with foundational clarity.1.5 Terms to Know (Modern Glossary)Primary Field: The magnetic field generated directly by the excitation coil.Secondary Field: The field produced by the induced eddy currents in the test object.Impedance Plane: A 2D plot of coil resistance (X-axis) vs. reactance (Y-axis), used to visualize changes in material or flaw response.Modulation: Any change in coil signal caused by material variation or flaw.Skin Effect: The tendency of eddy currents to concentrate near the surface at higher frequencies.By 1953, eddy current testing had moved beyond theory and laboratory demonstrations into widespread industrial application. At the forefront of this transition was the Magnatest line of instruments, built on principles developed by Dr. Friedrich Frster. Originally conducted at the Kaiser Wilhelm Institute in Germany as early as 1935, Frsters research into electromagnetic material characterization laid the groundwork for highly sensitive eddy current systems capable of detecting flaws, verifying alloy composition, and measuring conductivity.As William A. Cannon Jr. documented in his presentation at the 1953 ASNT Annual Meeting, the Magnarest FM-100 could directly read %IACS conductivity using a bridge circuit tuned by an oscillator and detector coil, calibrated against standard samples. It provided instant, non-contact conductivity measurementssomething unheard of just a few years earlier.The measurements are independent of the thickness of the material tested, provided that such material is thicker than the depth of penetration of the eddy currents induced by the detector. W.A. Cannon, 1953These instruments, though powered by vacuum tubes and analog dials, represented an early convergence of engineering and inspection automationforeshadowing the direction Hugo Libby and others would take in the decades ahead.In 1954, Argonne National Lab published an eddy current diameter gauge design that relied heavily on impedance plane theory developed by physicist R. Hochschild (NYO-3576).Hochschilds analysis of how conductivity, diameter, and frequency affect test coil behavior became a standard reference used to calibrate coil sensitivity, phase response, and signal conditioning.While Libby never co-authored a report with Hochschild, theres strong evidence that Libbys analog phase discrimination techniques especially those published in his 1959 and 1961 broadband reports were directly built on Hochschilds impedance plane foundations.Together with Frsters vector loop theory and Dodd & Deeds field solutions, Hochschild provided the mathematical spine of American ECT design a foundation Libby transformed into real-world instruments used across the nuclear industry.In 1954, physicist R. Hochschild published a deceptively titled paper: The Theory of Eddy Current Testing in One (Not-So-Easy) Lesson. It was, in fact, one of the most significant mathematical foundations ever written for ECT.Hochschilds work explained how coil impedance varied with material conductivity, geometry, and frequency using Bessel function models, normalized impedance, and phasor plots. His derivation of how resistance and reactance could be decoupled in flaw detection became the basis of Libbys later multi-parameter test systems. In his 1954 paper, Hochschild introduced many of the same concepts later expanded by Libby, including:Normalized R-X signal plotsUse of eddy current phase angle to distinguish flaw typesGradient detection using differential coilsInterpretation of flaws, wall thinning, and conductivity effects on the impedance planeHochschild credited Dr. Frster as the source of these impedance diagrams and Libby would later carry them forward into multiparameter signal discrimination systems for Hanford and beyond.Libby references this paper multiple times in his own book and Hanford reports. Although they never co-authored a paper, Libbys innovations especially the idea of separating flaw signals by rotating phase vectors can be seen as a practical evolution of Hochschilds mathematical insight.Before Hochschild or Libby refined eddy current flaw detection, Friedrich Frster laid the groundwork with his development of field strength and field gradient probes.In his 1955 paper, A Method for the Measurement of DC Magnetic Fields and DC Field Differences, Frster explained how opposing-coil arrangements could:Cancel out ambient fields like Earths magnetismDetect field gradients between probe halvesQuantitatively measure coercive force, residual magnetization, and defect leakage fieldsEnable wall thickness measurement and nondestructive grain texture analysisHis Frster Probe became a staple in magnetic field analysis and set the stage for the dual-channel signal strategies used by Libby.While Hugo Libby was pioneering phase-sensitive impedance analysis, William A. Cannon Jr. was demonstrating how electrical conductivity could be measured directly, quickly, and nondestructively using the Magnaflux FM-100.In this 1955 article, Cannon outlines how conductivity became a practical quality control parameter across multiple electrical applications:Sorting aluminum rotor castings by alloy type (2545% IACS) to prevent mismatched torque characteristics in AC motorsVerifying electrode materials in resistance welders to ensure optimal performance and avoid overheatingMonitoring as-cast conductivity in high-purity copper and aluminum alloys for critical electrical partsDetecting surface skins and heat-treatment zones through changes in IACS responseControlling melt chemistry by inferring phosphorus or oxygen content from conductivity drop-offsOne particularly powerful observation:By scanning the surface with the Magnatest probe, it was possible to detect grain boundaries in large-cast copper alloysrevealing directional conductivity variations due to crystallographic orientation.The FM-100 offered something Libby was also working toward: repeatable, quantifiable inspection results tied to meaningful material properties. While Libbys work explained why signals changed, Cannons tools made those changes measurable on the shop floor.These stories show that, by the mid-1950s, the theory behind eddy current testing was no longer trapped in academiait was starting to transform manufacturing practices.By 1955, eddy current theory was no longer just academic. In a paper presented by Lee A. Cosgrove of Alcoa, the Magnatest Conductivity Meter was showcased as a transformative tool for real-time quality control in aluminum processing.The instrument applied a 60 Hz current to a probe coil, which induced eddy currents in the test sample. The resulting impedance change in the coil, caused by variations in the materials conductivity, was measured via a bridge circuit and shown directly on a dial in % IACS.The measurement now requires only a few seconds and a flat area on the specimen equal to or greater in size than a nickel.Cosgrove described practical uses that echoed Libbys theoretical predictions:Sorting mixed alloys by conductivityDetecting improper heat treatment via subtle changes in conductivityFinding soft spots or heat-treat inconsistencies across large aluminum platesTracking operator-induced variation, like uneven furnace agingDetecting early fatigue damage through conductivity loss at the crack originEstimating chemical composition (e.g., silicon or iron content in aluminum)The key insight: conductivity was an indirect fingerprint of mechanical propertiesnot perfect, but good enough to trigger deeper inspection or rework.Cosgroves team once sorted 1,000 aluminum extrusions in under an hour, and separated 10 tons of defective sheet into good/bad batches in just 3 hourssomething that wouldve required destructive tensile testing on 1,600 samples otherwise.These real-world successes demonstrated that Libbys vision of signal interpretation was not only validit was transforming industry.Yes, Hugo Libby’s 1956 paper titled “Basic Principles and Techniques of Eddy Current Testing” is a cornerstone document for your Libby eBook—and it should be used as a featured chapter or Section F. This paper distills Libby’s theoretical understanding into practical inspection logic, offering both a historical narrative and working formulas that help technicians interpret coil behavior, signal interaction, and flaw detection.How to Use This in Your Book:✅ Direct Inclusion as Section F (Recommended Title):Section F – Libby’s 1956 Address: Basic Principles and Techniques of Eddy Current TestingYou may introduce it with a brief preface:This address, presented by H.L. Libby at the 16th Annual Convention of the Society for Nondestructive Testing in Cleveland, Ohio (1956), captures his transition from theoretical physicist to practical engineer. In this lecture, Libby demystifies the eddy current signal using a combination of energy-plane logic, skin-depth analogies, and probe performance curves. It remains one of the clearest articulations of how electromagnetic principles were applied to make eddy current testing not just possiblebut operationally reliable.Key Contributions to Highlight:Standard Depth of Penetration Formula:=1(or=1f)\delta = \sqrt{\frac{1}{\pi \mu \sigma}} \quad \text{(or } \delta = \frac{1}{\sqrt{\pi f \mu \sigma}} \text{)}=1(or=f1)This matches the form used today and is connected to Libbys analogies to thermal diffusion.Energy Plane and Impedance Plane Equivalence:Libby shows how voltage vectors, phase shift, and magnitude from eddy current probes relate to stored vs. dissipated energy, leading to:Phase-angle interpretationDefect sizingFrequency optimizationComparison of test conditions via locus curves on a complex plane (Figures 510), many of which are still taught today as classic cases.Scanning logic for subsurface defects, including diagrams (e.g., Figures 8 & 9) showing probe motion and signal rotation during flaw traversalideal for illustrating crack detection, liftoff, and material transitions.Coil Geometry Effects: Discussion of probe motion loci, shape effects, and multilayered materials (Figure 10), aligning directly with modern array probe development.Balance Circuitry and Phase-Amplitude Detectors: Diagrams (Figures 1115) illustrate how to suppress liftoff noise and emphasize flaw signalsprecursors to modern filter logic and software tools.