But 7 mil wires were somewhat harder to find. Filament current is about 5 A, and the filament voltage is about 10 V or less.
L is the length.
The emission current is a function of temperature, and is furthermore proportional to the filament surface area. So, the filament current does not change much over a wide range of emission current. However, small filaments as the ones that we use are sometimes run close to the space charge limit, and the behavior can be very different. One should try to avoid this limit, because the filament can be easily burnt out under feedback control. To avoid the space charge limit, use a higher accelerating voltage, or use a longer or thicker filament to increase the emission area.