and r2 these voltage drops are proportional to the direct current in the subscriber line 30 the series connection of the resistors r4 and r5 serve as a voltage divider for providing a voltage proportional to the voltage at node 46 to the positive input terminal of the amplifier 41 similarly the series combination of resistors r3 r6 and r7 provide a voltage proportional to the voltage at node 48 to the negative input terminal of the amplifier 41 the difference in voltages between the two input terminals of the amplifier 41 is a measure of the voltage drops across the resistors r1 and r2 and accordingly of the current provided by the battery 38 the resistor r8 provides negative feedback to the amplifier 41 and a fixed amount of gain based on the magnitude of resistance of the various resistors as is well known the resistor r7 is of variable resistance and is manually adjustable to set a desired level of zero input voltage across the input terminal of the amplifier 41 the resistor r7 is used therefore for initial alignment of the interface circuit 20 the capacitor c1 serves to bypass to ground any voice and data signals which may be received by the amplifier 41 from the primary windings l2 and l3 thereby the capacitor.