Ionization growth rates of hydrogen, in the range of $\frac{E}{p}$ between 100 and 10 000 V/cm Torr are presented. They have been deduced from microwave measurements of electron densities in a ring-shaped discharge, produced by a uniform magnetic field linearly increasing in time. The experimental conditions are described and the meaning of the obtained data is discussed as well as compared with the more usual results in a straight uniform electric field. At $\frac{E}{p}<~1000$, the obtained values of $\frac{\beta }{p}$, where $\beta$ is the ionization growth rate, increase with $\frac{E}{p}$ and are in good agreement both with theory and earlier experiments. At $\frac{E}{p}>2000\mathrm{}$, on the contrary, $\frac{\beta }{p}$ decreases with increasing $\frac{E}{p}$ as suggested by theory, but not yet measured experimentally. The maximum of $\frac{\beta }{p}\simeq 1.3\times {10}^9$ sec ${\mathrm{Torr}}^{-1\mathrm{}}$ corresponds to $\frac{E}{p}\simeq 2000$.

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