Characteristic impedance of vacuum
UniversalMeasured
= 376.730 313 412(59) Ω
- Value
- 376.730 313 412 Ω
- Standard uncertainty
- 5.9 × 10−8 Ω
- Relative uncertainty
- 1.6 × 10−10 (157 parts per trillion)
- Status
- Measured since 2019 (exact before)
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
# characteristic impedance of vacuum, CODATA 2022 (NIST)
characteristic_impedance_of_vacuum = 3.76730313412e2 # ohm, standard uncertainty 5.9e-8
# SciPy: scipy.constants.physical_constants['characteristic impedance of vacuum']What it is
The ratio of the electric to the magnetic field strength in an electromagnetic wave travelling through vacuum, about 377 ohms. Antenna engineers meet it as the impedance of free space.
Until CODATA 2014 this value was exact by definition. Since the 2019 SI redefinition it is measured, because the constant that used to fix it was replaced by a fixed elementary charge.
Equations
- definition
How the value changed, 1998–2022
Each point is one CODATA adjustment, plotted as its difference from the 2022 value; the bars show one standard uncertainty (an arrowhead means the bar runs off the chart).
Relative uncertainty
| CODATA | Value (uncertainty) | Relative uncertainty | Change from previous |
|---|---|---|---|
| 2022 | 376.730 313 412(59) | 1.6 × 10−10 | −4.5 σ of 2018 |
| 2018 | 376.730 313 668(57) | 1.5 × 10−10 | |
| 2014 | 376.730 313 461… | exact | exact |
| 2010 | 376.730 313 461… | exact |
“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).