33 values, each with its units, its uncertainty, and where it came from.
All 351 constants →
J/TThe natural quantum of magnetic moment for an electron, eħ/2mₑ — the yardstick for atomic magnetism and electron spin.
ΩThe ratio of electric to magnetic field strength in a plane wave in free space — the 377 ohms every antenna engineer matches to.
SThe conductance 2e²/h of a single ballistic quantum channel, about 77.5 μS — the step size in nanoscale wires and atomic point contacts.
N·m²/C²The proportionality constant 1/(4πε₀) in Coulomb's law, fixing the enormous strength of the electrostatic force between charges.
TTypical magnitude of the geomagnetic field at the surface, near 50 μT but ranging from about 25 μT at the equator to 65 μT near the poles.
C/kgThe electron's charge divided by its mass, −1.758 820 008 × 10¹¹ C/kg — the quantity J. J. Thomson measured in 1897 to discover the electron.
s⁻¹·T⁻¹The electron's spin precession rate per tesla, 658 times the proton's — the basis of electron spin resonance and of spin-qubit control.
J/TThe magnetic moment of a free electron, negative because its charge is, and about 0.116% larger than one Bohr magneton.
CThe charge of a proton, exactly 1.602 176 634 × 10⁻¹⁹ C — the quantum of free charge and the SI definition of the ampere.
C/molThe charge carried by one mole of electrons, N_A×e ≈ 96485 coulombs — the bridge between the coulombs you meter and the moles you plate.
ΩHalf the von Klitzing constant, h/2e² ≈ 12.906 kΩ — the resistance of one perfect ballistic channel and the floor for any nanoscale wire.
Hz/VThe frequency-to-voltage ratio 2e/h of a Josephson junction — 483.6 THz per volt, and the modern practical realisation of the volt.
WbThe smallest unit of magnetic flux that can thread a superconducting loop, h/2e — exact since the 2019 SI redefinition fixed h and e.
HzNominal 50 Hz AC power frequency used across Europe, Asia, Africa and Oceania, regulated to roughly ±0.05 Hz in normal grid operation.
HzNominal 60 Hz AC power frequency across North America, held within about ±0.05 Hz by grid operators balancing generation against load.
J/TA neutral particle with a magnetic moment of −1.913 nuclear magnetons — proof on its own that the neutron has charged internal structure.
VNominal 230 V RMS single-phase supply under IEC 60038, the harmonised European figure that replaced the old 220 V and 240 V standards.
VNominal 120 V RMS at North American outlets, with ANSI C84.1 allowing roughly 114–126 V at the point of utilisation under normal service.
J/TThe magnetic-moment unit for nuclei, eħ/2mₚ — smaller than the Bohr magneton by the full proton-to-electron mass ratio of 1836.
s⁻¹·T⁻¹The proton's precession rate per unit magnetic field in angular frequency — the constant that turns a magnet strength into an NMR frequency.
Hz/TThe proton Larmor frequency per tesla, 42.577 MHz/T — the number every NMR spectroscopist and MRI physicist works in directly.
J/TThe magnetic moment of the proton, 2.79 nuclear magnetons rather than the 1 a point particle would show — evidence of quark structure.
—Typical maximum relative permeability of commercial soft iron — a wildly variable figure spanning roughly 200 to 5000 with purity and field level.
—Typical relative permeability of annealed mu-metal in weak fields — the nickel-iron alloy used to shield instruments from stray magnetic fields.
—Typical dielectric constant of dry air at 0 °C and one atmosphere — so close to vacuum that most capacitor and antenna work ignores the difference.
—Typical dielectric constant of FR-4 circuit-board laminate near 1 GHz — the number behind every microstrip impedance and trace-delay calculation.
—Typical dielectric constant of PTFE (Teflon), about 2.1 and almost flat from DC to tens of gigahertz — the benchmark low-loss RF insulator.
—Typical static dielectric constant of liquid water at 20 °C — an outlier among common liquids and the reason water dissolves salts so well.
m/sTypical propagation speed in crown glass, c/1.52 or about 197,000 km/s — the delay that makes lenses focus and optical fibres carry data.
m/sLight travels through water at about 225,000 km/s, or c/1.333 — the slowing that bends a straw at the waterline and lets Cherenkov detectors work.
F/mThe electric constant of free space, setting the strength of Coulomb's law and the capacitance of every parallel-plate geometry.
N/A² (H/m)How strongly a current magnetises empty space — the constant in Ampère's law, no longer exactly 4π×10⁻⁷ since the 2019 SI redefinition.
ΩThe quantum Hall resistance h/e² ≈ 25.813 kΩ, exact since 2019 and the reference by which the ohm is now realised worldwide.
