Plot showing the effect of increasing electron fractional abundance (thick black curves, calculated using a constant kinetic temperature of 12 K) and kinetic temperature (dashed curves, calculated using a negligible electron abundance) on the 4  →  3/1  →  0 line intensity ratios of H¹³CO⁺ and HN¹³C, for gas with an H₂ density of 6 × 10⁵ cm^-3 and H¹³CO⁺ and HN¹³C column densities of 8.8 × 10¹¹ cm^-2 and 5.1 × 10¹¹ cm^-2 respectively. The grey bar shows the observed H¹³CO⁺ 4  →  3/1  →  0 line intensity ratio towards NGC 1333, taking into account the 1σ errors in the observations. The thin black line shows the upper limit to the observed HN¹³C 4  →  3/1  →  0 line intensity ratio towards NGC 1333.