  We evaluate the linearity of three electric measurement techniques of the
initial electron density in laser filaments by comparing their results for a
pair of filaments and for the sum of each individual filament. The conductivity
measured between two plane electrodes in a longitudinal configuration is linear
within 2% provided the electric field is kept below 100 kV/m. Furthermore,
simulations show that the signal behaves like the amount of generated free
electrons. The slow ionic current measured with plane electrodes in a parallel
configuration is representative of the ionic charge available in the filament,
after several $\mu$s, when the free electrons have recombined. It is linear
within 2% with the amount of ions and is insensitive to misalignment. Finally,
the fast polarization signal in the same configuration deviates from linearity
by up to 80% and can only be considered as a semi-qualitative indication of the
presence of charges, e.g., to characterize the filament length.
