[en] A voltmeter of sensitivity 2 x 10^-15V/√Hz(2 fV/√Hz) is based on direct conversion of the voltage to be measured into a frequency according to the Josephson voltage-frequency relation. Self-pumped parametric amplification in a d.c. biased loop containing two Josephson junctions and a measuring resistance Rsub(s) = 4.7 x 10^-9 Ω amplifies the femptovolt level voltages across Rsub(s) to microvolt levels that can be observed with a room temperature amplifier. The sensitivity of the femptovoltmeter is determined by Johnson-Nyquist noise voltages at 4.2 K in the 4.7 x 10^-9 Ω resistance. (Auth.)