critical current of the dc SQUID changes in the interval $\Delta \Phi = \Phi _{0}/2$. Consequently the maximum value $(\partial V/\partial \Phi ) _{I}$ of the classical dc SQUID cannot exceed $2\Delta /e\Phi _{0}$. The real value $(V/\Phi ) _{I} \approx 2 \ \mu V/\Phi _{0}$ [@SQUID1977] of a typical dc SQUID is substantially smaller than the maximum value. In this work we explore an alternative device for the measurement of the weak magnetic filed, a differential double contour interferometer (DDCI). The idea of this new device arose thanks to the experiment made in [@Zhilyaev2000] and its explanation [@Zhilyaev2001].

Higher sensitivity of DDCI compared to the conventional SQUID is provided by strong discreteness of the energy spectrum of the continuous superconducting loop [@PLA2012QF]. According to the conventional theory [@Tink75] the total energy of the persistent current $$I_{p} = \frac{n\Phi_{0} - \Phi }{L_{k}} \eqno{(1)}$$ in a loop with small cross section $s \ll \lambda _{L}^{2}(T)$ is determined mainly by the kinetic energy [@QuSMF2016]: $$E _{t} \approx E _{k} = \frac{L_{k}I_{p}^{2}}{2}= \frac{\Phi _{0}^{2}}{2L_{k}}(n - \frac{\Phi }{\Phi _{0}})^{2} \eqno {(2)}$$ Here $L _{k} = ml/ q^{2}n_{s}s = (\lambda _{L}^{2}/s) \mu _{0}l \approx (\lambda _{L}^{2}/s)L$ is the kinetic inductance of the loop of side 