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# Peek's law

In physics, Peek's law is a description of the conditions necessary for corona discharge between two wires: $e_v = m_v g_v \delta r \ln \left ({S \over r} \right )$

ev is the "visual critical corona voltage" or "corona inception voltage" (CIV), the voltage (in kilovolts) required to initiate a visible corona discharge between the wires.

mv is an irregularity factor to account for the condition of the wires. For smooth, polished wires, mv = 1. For roughened, dirty or weathered wires, 0.98 to 0.93, and for cables, 0.87 to 0.83.

r is the radius of the wires

S is the distance between the wires

δ is the air density factor. It is calculated by the equation: $\delta = {3.92 b \over 273 + t}$
where
• b = pressure in centimeters of mercury
• t = temperature in degrees Celsius
At SATP (25°C and 76 cmHg): $\delta = {3.92\cdot76 \over 273 + 25} = 1$

gv is the "visual critical" potential gradient, and is calculated by the equation: $g_v = g_0 \delta \left ( 1 + {0.301 \over \sqrt{\delta r}} \right )$
where g0 is the "disruptive critical" potential gradient, about 30 kV/cm for air 

## References

1. ^ Hong, Alice (2000). Electric Field to Produce Spark in Air (Dielectric Breakdown). The Physics Factbook.
• F.W. Peek (1929). Dielectric Phenomena in High Voltage Engineering. McGraw-Hill.
• High Voltage Engineering Fundamentals, E.Kuffel and WS Zaengl, Pergamon Press, p366