measurement of frequency

An unknown frequency source signal generator is connected to y plates of C.R.O . Time base signal is connected to x plates internally connected . We get a sinusoidal wave on the screen, after the adjustment of voltage sensitivity band switch Y plates and time base band switch X plates . The horizontal length l between two successive peaks is noted. When this horizontal length l is multiplied by the time base m i.e. sec div , we get the time period T .The reciprocal of the time period 1 T gives the frequency f . This can be verified with the frequency, measured by the multi meter.