R+△R1R2-△R2 UO R3一△R3R4+△R4 U=III R R R-RR R1+R2R3+R4(R+R2)(R3+R4) dU △0≈=C△AR+0△R2+△R3+C△R4 R R R dRa r,R △R,△R△R,△R U( +-)(△R<<R) (R1+R2)2R1R2R3R UAR,△R,△R2△R 4R R2 R3 RU U0 R1+△R1 R2－△R2 R3－△R3 R4+△R4 1 3 1 4 2 3 0 1 2 3 4 1 2 3 4 0 0 0 0 0 1 2 3 4 1 2 3 4 1 2 1 2 4 3 2 1 2 1 2 3 4 1 2 4 3 1 2 3 4 ( ) ( )( ) ( ) ( ) ( ) ( ) 4 i i R R R R R R U U U R R R R R R R R dU dU dU dU U R R R R dR dR dR dR R R R R R R U R R R R R R R R U R R R R R R R R − = − = + + + +    +  +  +       − − +   +     = − − +