Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

212 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSstructure of the am, such as changes in the lattice constant due to penetrationof the adsorbate or local sintering due to a high heat of adsorption.The simplest method of measuring conductivity is to determine the voltagedrop across the sample and the current through the sample. The conductivity(a ohm-'. cm-') is then given byu = IL/VAwhere I is the current in amperes, V is the voltage drop, and L and A. are thelength and the cross sectional area of the specimen respectively. A refinementof this technique is the potential probe measurement. This technique uses apotentiometer which draws negligible current to measure the potential drop acrosspart of the specimen. L in the above equation then becomes the distance betweenthe potential probes. Under these conditions, the effect of contact resistances isminimised. An electrometer can be used where the circuit has a high impedance'46.Bridge methods are also a standard technique for conductivity measurements.The basic Wheatstone bridge circuit is illustrated in Fig. 18. When the bridge isbalanced, i.e. the detector D indicates zero current, then the unknown resistanceR, is given byOne important deficiency of the Wheatstone bridge is that it measures both sampleand contact resistance. Thus it is particularly inadequate for measurements below10 ohms. For measurements on metallic specimens with low resistances down toFig. 18. Basic Wheatstone bridge circuit for the measurement of electrical conductivity.