Conventional Bridge Measurements
M. H. W. Gall · 1972
The Wheatstone bridge is the most convenient instrument for measuring resistance. In its basic form it consists of four resistors forming a square as shown in Fig.l. It is easy to see that if a voltage is applied to the points A & B that the volt drop along the upper arm at the point C will be the same as the volt drop along the bottom arm at the point D if the ratio of Rl to R2 is the same as the ratio of R3 to R4. That is R1/R2 = R3/R4. It follows therefore that if a galvanometer is connected between C and D it will indicate zero current if these conditions are satisfied. That is the bridge is in balance when R1/R2 = R3/R4.