# Peek's law

In physics, Peek's law defines the electric potential gap necessary for triggering a corona discharge between two wires:

$e_v = m_v g_v r \ln \left ({S \over r} \right )$

ev is the "visual critical corona voltage" or "corona inception voltage" (CIV), the voltage 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, namely the surface irregularities result in diminishing the corona threshold voltage.

r is the radius of the wires in cm.

S is the distance between the wires

δ is the air density factor with respect to SATP (25°C and 76 cmHg):

$\delta = {\rho \over \rho_{SATP}}$

gv is the "visual critical" electric field, and is calculated by the equation:

$g_v = g_0 \delta \left ( 1 + {c \over \sqrt{\delta r}} \right )$
where g0 is the "disruptive electric field", and c is an empirical dimensional constant. The values for those parameters are usually considered to be about 30-32 kV/cm (in air [1]) and 0.301 cm½ respectively. This latter law can be considered to hold also in different setups, where the corresponding voltage is different due to geometric reasons.

## References

1. ^ Hong, Alice (2000). "Electric Field to Produce Spark in Air (Dielectric Breakdown)". The Physics Factbook.