Tap the jockey down at a few places along the wire and watch the galvanometer.
01Look at the circuit before you touch it: unknown in one gap, standard in the other, galvanometer across the middle.The bridge compares two ratios. Knowing which resistance is in which gap is what tells you which way to move when the needle kicks.
02Tap the jockey near one end of the wire and note which way the needle goes.Starting at an end guarantees a large, unambiguous kick. Starting in the middle can leave you inside the dead band with nothing to go on.
03Tap near the other end, and check the needle kicks the OTHER way.That pair of opposite kicks is the bracket. Until you have it you don’t know the null is on the wire at all, let alone where.
04Tap in the middle of the interval, and keep whichever half still has a sign change in it.Each middle tap halves the interval. This is the fastest search there is, and at every stage you can state your uncertainty as the width of what is left.
05Stop when the needle no longer moves either side of your position.That dead spot is the galvanometer’s sensitivity, and it’s the real limit of the method. Its middle is the balance point; claiming more precision than its width is claiming more than the instrument gave you.
06Read the balance point off the metre rule, not off the galvanometer.The whole reason for a null method is that a length on a rule can be read far more precisely than a needle on a scale. Writing down a deflection would throw that away.
07Tap briefly each time, and lift the jockey between taps.A jockey held down heats the wire and scrapes it. Both change the resistance you’re in the middle of measuring.
Laboratory
Move the jockey and tap it down. Do it at three or four places, well spread out.