Mu-metal was developed by Willoughby S. Smith and Henry J. Garnett and patented in 1923 for inductive loading of submarine telegraph cables; Telcon made 30 tons per week in its first year.
The roughly 80% nickel, 12–15% iron, 5% molybdenum alloy reaches relative permeabilities of 80,000–100,000, versus several thousand for ordinary steel.
It shields sensitive equipment from static and low-frequency magnetic fields in applications from CRT oscilloscopes and photomultiplier tubes to hard-drive magnet backings and MRI equipment.