. A text-book of electrical engineering;. Bridge. The measurement of resistance with the Wheatstone bridge is an import-ant and instructive apphcation of Kirchhoffs rules. The cell E (Fig. 14), whose need neither be known nor constant,is connected across the ends of a bare straight wire AB. Under the wire is a scale by means of which theexact position of the sliding con-tact C can be read. Connectedin parallel with this wire is abranch consisting of the un-known resistance x, the value ofwhich is to be determined, anda known resistance R. The re-sistance of the connecting wiresis assume
. A text-book of electrical engineering;. Bridge. The measurement of resistance with the Wheatstone bridge is an import-ant and instructive apphcation of Kirchhoffs rules. The cell E (Fig. 14), whose need neither be known nor constant,is connected across the ends of a bare straight wire AB. Under the wire is a scale by means of which theexact position of the sliding con-tact C can be read. Connectedin parallel with this wire is abranch consisting of the un-known resistance x, the value ofwhich is to be determined, anda known resistance R. The re-sistance of the connecting wiresis assumed to be galvanometer is connectedbetween the sliding contact C andthe connecting point between xand R. The sliding contact ismoved until no current flowsthrough the galvanometer. Thecurrent J^ then flows uniformly through the resistances a and h, and thecurrent I^ through the resistances x and R. We now apply Kirchhoffs * For further information on this subject, see The Grouping of Electric Cells, byW. F. Dunton. Spon. 8. Wheatstones Bridge 17 second rule to the left-hand lower mesh, going round it in a clockwisedirection as indicated by the dotted arrow. This circuit contains no the current in the galvanometer is zero; therefore I^a + 0 — I^x = in the right-hand mesh I-fi - I^R +0 = 0. Ct X Hence it follows that a^p (^4)- The ratio a: b in this equation refers to the resistances of the two portionsof the slide wire; these are, however, proportional to their lengths, so thatthe ratio a: b can be read off directly on the scale. It is instructive to deduce this result, without using Kirchhoffs rules,from the analogy of a water main which divides into two parallel verticalmains. If a horizontal pipe joins the two mains at any level, no water willflow through this connecting pipe, because there is no difference of level orpressure between its ends. This is exactly analogous to the Wheatstonebridge. When no current flows through the galvan
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