switches 11 14 15 18 and through inverters 19 20 21 22 to the control terminal of the switches 13 12 17 16 respectively the resistances of the resistors r1 r2 are equal the resistance of the resistor rk equals the doubled resistance of the load at the output o of the supply circuit sc for the moment the operation of the supply circuit sc in accordance with the invention will be represented schematically with fig3 a b alternately through the switches 9 10 the current i and i respectively is supplied to the multiplication circuit which is composed of a hall sensor 6 and of an operational amplifier a fig3 a resistors 7 8 symbolically represent oppositely equal resistances with an absolute value equal to the equivalent resistance of both the supply circuit and the multiplication circuit a negative impedance converter fig3 b composed of an operational amplifier a i 39 and of resistors ra rb transforms the resistance r m at the output into the resistance r m 39 r m u00b7 ra rb at the input if the resistors ra rb are connected as represented in fig3 b and into the resistance r m 39 r m u00b7 rb ra if the resistors in fig3 b are mutually interchanged if this interchanging is performed by a control signal p periodically with realization times of both configurations being equal then the input resistance is averaged to r m av 39 ra rb rb ra u00b7 r m 2 supposing that in monolithic technology two resistors with equal resistances differing for 1 can be made it follows from the last expression that the relation r m 39 r m is realizable with an accuracy of 0 01 at the output o of the circuit sc there flows the supply.