Fahy permeameter¹⁾ or Fahy's permeameter²⁾ - an apparatus (permeameter) used for characterisation of magnetically soft materials in shape of rods, bars and plates. Fahy permeameters are used mainly with DC magnetisation.³⁾

The device was invented by Frank P. Fahy of the Pennsylvania Railroad Co.⁶⁾

A patent for the first version was applied in 1912 (the patent was granted in 1916). The device comprised a double-sided magnetic yoke designed to compare the sample under test to a standard sample with known magnetic properties, and it was essentially a kind of magnetic comparator.⁷⁾

The yoke was designed with removable clamping elements, which could accommodate rod and bar samples of various cross-section shapes (round, square, etc.)

Fahy patented an improved version of the permeameter (patent granted in 1924). In this version the yoke was asymmetrical and did not require a reference sample for measurement, but only for calibration. The measurement employed the concept of magnetic potentiometer.⁸⁾⁹⁾

The single-sided version was later commonly referred to as Fahy Simplex permeameter.¹⁰⁾

Fahy permeameter was used commonly for testing magnetic materials, for instance by Norman P. Goss - the inventor of grain-oriented electrical steel.¹¹⁾

Fahy permeameter proved to have better measurement accuracy to Burrows permeameter, which was used as a standard measurement method before, and was more susceptible to the influence caused by non-uniformities in the sample under test.¹²⁾

Development of Fahy permeameter allowed more accurate magnetic measurements and eventually lead to international standards for magnetic testing of materials under DC magnetisation, with the Fahy permeameter being one of many similar devices used for the purpose of magnetic testing.¹³⁾

• Permeameter
• Permeability