A sonometer verifies the laws of vibrating strings: (1) Law of length: $f \propto 1/L$ (constant $T, \mu$). (2) Law of tension: $f \propto \sqrt{T}$ (constant $L, \mu$). (3) Law of mass: $f \propto 1/\sqrt{\mu}$ (constant $L, T$). Combined: $f = (1/(2L))\sqrt{T/\mu}$. The sonometer wire vibrates in resonance with a tuning fork when the natural frequency of the wire segment matches the fork frequency.