fT

fT, femtotesla, is the SI unit of magnetic flux density (magnetic field strength) equal to 10^-15 tesla. The symbol fT is used for reporting extremely small magnetic fields, particularly in high-sensitivity magnetometry and related fields.

In practice, fT-level measurements appear in areas such as magnetoencephalography (MEG), geophysics, and advanced laboratory measurements. Neuromagnetic signals detected noninvasively in MEG are typically on the order of tens to hundreds of femtotesla, while geomagnetic and laboratory fields span much larger ranges. For reference, the Earth's magnetic field is about 50 microtesla (4–5 × 10^10 fT), illustrating the wide dynamic range involved in precision measurements.

Detection of fT-level fields relies on highly sensitive instruments, especially superconducting quantum interference devices (SQUIDs) and advanced atomic magnetometers. Such sensors often operate in magnetically shielded environments to reduce noise, achieving noise floors on the order of a few femtotesla per root hertz in favorable conditions. Ongoing advancements aim to improve sensitivity, bandwidth, and practicality for clinical and scientific use.

Conversions and relationships: 1 fT = 10^-15 T. Other nearby units include 1 pT = 10^-12 T and 1 nT = 10^-9 T. The usage of fT emphasizes the extreme weakness of certain measured magnetic fields and the corresponding requirements for ultra-sensitive instrumentation.

See also: Tesla; magnetometer; SQUID; geomagnetism.