D.C. Circuits 65 From the above example it may be appreciated that due to the ratio arms, the Wheatstone Bridge is capable of measuring a very wide range of resistance values. The instrument is also very accuratebecause it is what is known as a null method of measurement. This term is used because no settings on the three arms are used to determine the value of Rx until the galvo (G) in the central limb indicates zero (null reading). Since the galvo is a very sensitive microammeter it is capable of indicating fractions of a microamp. Hence, the slightest imbalance of the bridge can be detected. Also, since the bridge is adjusted until the galvo indicates zero, then this condition can be obtained with maximum accuracy. The reason for this accuracy is that before any measurements are made (no current through the galvo) it is a simple matter to ensure that the galvo pointer indicates zero. Thus, only the sensitivity of the galvo is utilised, and its accuracy over the remainder of its scale is unimportant. Included in the central limb are a resistor and a switch. These are used to limit the galvo current to a value that will not cause damage to the galvo when the bridge is well off balance. When the ratio arms and the variable arm have been adjusted to give only a small deﬂ ection of the galvo pointer, the switch is then closed to bypass the swamp resistor Rs . This will revert the galvo to its maximum sensitivity for the ﬁ nal balancing using Rv . The bridge supply is normally provided by a 2 V cell as shown. Do not confuse accuracy with sensitivity. For an instrument to be accurate it must also be sensitive. However, a sensitive instrument is not necessarily accurate. Sensitivity is the ability to react to small changes of the quantity being measured. Accuracy is to do with the closeness of the indicated value to the true value 2.10 The Slidewire Potentiometer This instrument is used for the accurate measurement of small voltages. Like the Wheatstone Bridge, it is a null method of measurement since it also utilises the fact that no current can ﬂ ow between points of equal potential. In its simplest form it comprises a metre length of wire held between two brass or copper blocks on a base board, with a graduated metre scale beneath the wire. Connected to one end of the wire is a contact, the other end of which can be placed at any point along the wire. A 2 V cell causes current to ﬂ ow along the wire. This arrangement, including a voltmeter, is shown in Fig. 2.31 . The wire between the blocks A and B must be of uniform cross-section and resistivity throughout its length, so that each millimetre of its length has the same resistance as the next. Thus it may be considered as a number of equal resistors connected in series between points A and B. In other words it is a continuous potential divider.