The History and Development of the Rogowski Coil (1912–1950)

The Early 20th-Century Industrial Context

In the early 20th century, the electrical industry experienced rapid growth, spurred by widespread industrial electrification, the expansion of power distribution networks, and advancements in high-voltage systems. Accurate current measurement was critical for power generation, distribution, and equipment testing. At the time, current transformers (CTs) represented the state-of-the-art solution for measuring electrical power. They offered reliable performance, practical integration into electrical systems, and a proven method for isolating measurement equipment from high-voltage circuits.

CTs excelled in many applications due to their simplicity, robustness, and ability to handle significant current magnitudes without requiring invasive techniques. They converted high primary currents into manageable secondary currents for instrumentation and protection systems, playing a central role in power system operations—an importance they retain to this day.

However, CTs also had inherent limitations that became apparent as the industry’s requirements evolved:

• Core Saturation: The magnetic cores of CTs introduced nonlinearities at very high current levels or during transient events, reducing their accuracy under such conditions.
• Frequency Dependence: CTs were optimised for standard power system frequencies (e.g., 50 or 60 Hz) but struggled with accuracy at very high or low frequencies, limiting their applicability in emerging fields like radio-frequency and transient analysis.
• Size and Weight: Magnetic cores made CTs relatively bulky, which posed challenges for compact systems or mobile applications where weight and space were at a premium.

Despite these limitations, CTs remained indispensable due to their practicality and performance in most power-related applications. Yet, as the electrical landscape expanded into more specialised and demanding areas, the need for alternative solutions that offered high accuracy, linearity, compactness, and wider operational flexibility became increasingly clear.

The Rogowski coil emerged as a promising innovation to address these challenges while complementing the strengths of existing technologies like CTs.

Rogowski’s ”Magnetic Voltage Coil”

The 1912 paper by W. Rogowski and W. Steinhaus, "The Measurement of Magnetic Tension," which laid the theoretical and experimental foundation for the Rogowski coil, originally referred to it as the “Magnetic Voltage Coil”, a device designed to directly measure the line integral of magnetic field strength. 

This innovative approach bypassed the core-related limitations of existing technologies, particularly magnetic saturation, and hysteresis, and provided a more linear response across varying current magnitudes.

Theoretical Contributions

Rogowski and Steinhaus presented the theoretical basis for their device by leveraging Maxwell’s equations, which relate the circulation of the magnetic field to the enclosed current. They proposed using a uniformly wound helical coil to measure this circulation, with the output voltage directly proportional to the rate of change of the enclosed current (di/dt).

Key aspects of their theory included:

• Coreless Design
  Eliminating the magnetic core ensured linearity, immunity to saturation, and removed distortion due to hysteresis.
• Output Proportionality
  The induced voltage was proportional to di/dt, making it ideal for capturing transient and alternating current behaviour.
• Calibration Potential
  The authors highlighted that the coil could be calibrated to accurately relate the induced voltage to the actual current, a feature essential for practical applications.

Practical Construction and Testing

To validate their theory, Rogowski and Steinhaus built a prototype magnetic voltage coil using a strip of pressed chipboard as the coil former. The strip was hand-wound with a uniform, helical, single layer of wire, where the ends were terminated for connection to a measurement instrument. Key details of their experimental work included:

• Winding Consistency
  Achieving uniform winding density was critical for accuracy. Despite their best efforts, some deviations were noted due to the manual construction process.
• Mechanical Assembly
  The coil was enclosed in protective rubber bands and mounted in various configurations to test its response to different magnetic fields.

Experimental Observations

Their experiments confirmed the theoretical predictions, demonstrating that the device could accurately measure the magnetic circulation voltage under various conditions. Key findings were:

• Accuracy in Closed-Loop Measurements
  When the coil completely encircled a current-carrying conductor, the measurements were consistent and proportional to the current.
• Sensitivity to Transient Currents
  The device’s output clearly reflected rapid changes in current, validating its suitability for transient analysis.
• Independent of Core Materials
  Unlike CTs, the coil’s performance was unaffected by the nonlinearities associated with magnetic cores.

Challenges and Limitations Identified

Despite the promising results, Rogowski and Steinhaus identified several practical challenges that needed to be addressed for wider adoption:

• Winding Imperfections
  Variations in winding density and former cross-section affected measurement accuracy.
• Integration Complexity
  The coil’s output, being proportional to di/dt, required external integration to reconstruct the current waveform—a non-trivial task given the limitations of early electronics.
• Environmental Sensitivity
  The device was susceptible to stray magnetic fields external to the coil, emphasising the need for shielding or compensatory designs.

Specific Applications

Despite its challenges, the Rogowski coil found niche applications that leveraged its unique advantages. Early uses included:

High-Voltage and High-Current Systems

• Use Case
  Measurement of large fault currents in power lines and substations.
• Value
  The coil’s coreless design avoided saturation, enabling accurate transient current measurements in scenarios involving lightning strikes or short circuits.

Transformer Core Loss Studies

• Use Case
  Measuring magnetic field distributions and losses in transformer cores.
• Value
  The Rogowski coil’s precision in measuring alternating current harmonics made it ideal for studying efficiency and material behaviour.

Impulse Testing

• Use Case
  Simulating and measuring the effects of lightning impulses on power equipment.
• Value
  Its high-frequency response allowed detailed analysis of impulse waveforms and stress distribution in insulation systems.

While these applications demonstrated the coil’s potential, limitations in integration and environmental sensitivity confined its usage to controlled laboratory environments

Development from 1912 to 1950

The decades following the coil’s introduction saw incremental improvements in design, manufacturing, and application techniques, like:

Winding Technology

Early coils were hand-wound, leading to inconsistencies. By the 1930s, machine-winding technologies allowed for greater uniformity, reducing errors.

Compensating Return Coil

In the 1930’s the concept of having a compensating ‘return’ conductor or coil, that went close to, or back down, the axial centre of the main coil winding was introduced. This greatly improved its ability to reject interference from magnetic fields outside of the coil loop.

Advancements in Integration

Vacuum-tube-based electronic integrators emerged in the 1940s, addressing the challenge of converting the coil’s output into a voltage proportional to current rather than di/dt. These developments enabled the coil’s application in dynamic and real-time systems.

Standardisation and Calibration

Calibration techniques improved with the development of standardised test currents and precision measurement equipment. This established the coil as a trusted tool in high-accuracy environments.

By 1950, the Rogowski coil had transitioned from a theoretical curiosity to a practical tool in niche but critical applications, laying the groundwork for its broader adoption in subsequent decades.

Conclusion 1912 - 1950

The Rogowski coil’s journey from its inception in 1912 to its incremental development by 1950 illustrates the balance between innovative potential and practical limitations. While the coil found application measuring transient and alternating currents, particularly in high-voltage and high-frequency applications, it faced challenges that limited its widespread use. In contrast, current transformers (CTs) remained the preferred technology in most electrical systems due to their robustness, ease of integration, and ability to deliver precise, calibrated measurements without the need for external integration. 

The CT's proven reliability and versatility in standard power systems ensured its dominance in practical applications. However, incremental advancements in winding techniques, integration, and calibration gradually expanded the Rogowski coil’s utility, carving out a complementary role in specialised and transient measurement scenarios. This progression underscores the coil’s enduring relevance as a valuable tool in the evolution of electrical measurement technology alongside the continued dominance of CTs in mainstream power systems.