The form factor F was introduced by J. Vermeer in the paper: 'A simple formula for the calculation of the convective heat transfer between conductor and sheath in compressed gas insulated (CGI) cables', 1983 as published in Elektra 87.

A different form factor - for single- and three-core GIL - was introduced by K. Itaka, T. Araki, and T. Hara in the paper: 'Heat Transfer Characteristics of Gas Spacer Cables', 1978.

$\frac{D_{\mathrm{c}} \left(- \frac{D_{\mathrm{c}}}{D_{\mathrm{comp}}} + 1\right)^{2.167}}{\ln{\left (\frac{D_{\mathrm{comp}}}{D_{\mathrm{c}}} \right )}}$Vermeer1983
$\frac{D_{\mathrm{c}}^{0.75}}{\left(\left(\frac{D_{\mathrm{c}}}{D_{\mathrm{comp}}}\right)^{0.6} + 1\right)^{1.25} \left(\ln{\left (\frac{D_{\mathrm{comp}}}{D_{\mathrm{c}}} \right )} + 2.2\right)}$Itaka1978, single-core
$\frac{3 D_{\mathrm{c}}^{0.75}}{\left(2.40822468528069 \left(\frac{D_{\mathrm{c}}}{D_{\mathrm{comp}}}\right)^{0.6} + 1\right)^{1.25} \left(\ln{\left (\frac{0.416666666666667 D_{\mathrm{comp}}}{D_{\mathrm{c}}} \right )} + 2.2\right)}$Itaka1978, three-core
Used in