ATM010, time‐resolved, cavity perturbation technique has been used to measure the total number of electronsNein a magnetic mirror compression experiment(Te ∼ 75 keV; ne ∼ 1012−1013 cm−3; plasma length∼ 10 cm; plasma diameter∼ 0.5 cm; cavity length∼30 cm; cavity diameter∼10 cm).Neis measured independently of the plasma density profile and in a high plasma density regime(&ohgr;p > &ohgr;010). High density measurements are possible because the plasma skin depth is greater than the plasma diameter and because the induced charge at the ends of the plasma is small. A swept frequency, raster presentation is used so that the variation ofNewith time can be followed. The measurements show thatNe ∼ 1012−1013at peak compression, and thatNedecays on a time scale∼4 msec. The cavity measurements were in substantial agreement with synchrotron radiation and light measurements, as well as with simple mirror theory.