(a) The Fall of Potential Method is well adapted to the requirements of practical electricians, because the only instruments needed are an ammeter and a voltmeter.
Demonstration. - Couple an ammeter A (Fig. 1) in series with the resistance R to be measured. To the terminals B and C of this resistance couple a voltmeter V, as shown. Make the. current by the switch S, and read both instruments at the same time. Substitute these readings in
derived from Ohm's Law, and the value of R is determined. It is to be observed that the reading of the ammeter gives the sum of the currents through the resistance R and through the voltmeter. On account of the high resistance of the voltmeter, the current will usually be so small that it may be neglected.
(b) The Method of the Wheatstone Bridge. - The voltmeter circuit from the point B to the point C in Fig. 1 is called a shunt circuit, or a parallel circuit.
Wheatstone made use of the fall of potential in parallel circuits for the measurement of resistance. When two points A and В (Fig. 2) are connected by two parallel circuits, the total fall of potential in the upper branch AxB is equal to the total fall in the lower branch AyB. There must be a point y in the lower branch where the fall of potential from A is exactly the same as the fall of potential in the upper branch from A to x; so that the points x and y will have the same potential. If, then, the points x and y are connected by a wire, no current will flow in the wire. If, however, a wire is connected from x to a point z between A and y, a current will flow from z to x. If the connection is made to a point w, between y and B, the flow will be from x to w. By the introduction of a galvanometer in the conductor xy, the proper position of y in which no current passes in xy can readily be determined, and when this is found, the resistance of Ax : the resistance of xB = the resistance of Ay : the resistance of yB.
In the slide wire Wheatstone bridge (Fig. 3) the circuit AYB is a single wire of uniform diameter; hence its resistance is proportional to its length, and the proportion becomes r:r' = l:l, in which r and r'are resistances and I and l' lengths. The key k is used to make contact with the wire in determining the position of Y. For XB is put the resistance box, while AX is the resistance to be measured.
FIG. 4.-A, Wheatstone Bridge; B, Battery; C, Galvanometer; D, Resistance Box; E, Battery Key; F, Galvanometer Key; G, Material to be tested; HH, Spools of proportional resistance
Figure 4 shows a form of Wheatstone bridge in which the slide wire is replaced by spools of resistance wire HH. These can be put into the circuit by taking out the short-circuiting plugs and any, or all of them, can be used.
