Terry 5. Carlton Oberlin College Oberlin, Ohio 44074
Conductance Experiments Utilizing a Wide-Range DC Meter
The inexpensive, semiquantitative conductancr meter described in this article is convenient and ruggcd cnough to be used by inexperienced students (see Fig. 1). I t has seven overlapping ranges of 1 to lCOO ma full scale. Using this meter one can measure the conductance of aqueoussolutions of strongelectrolytes, yet observe marked differencesbetm-een tap water, distilled water, and toluene. The meter is used in a set of experiment~which emphasize that conductance is a measure of ion concentrations. In two of the experiments conductance measurements are applied in investigations of acid-base reactions in which the solvent is the base.
The circuit diagram for the meter is given in Figure 2, and the parts list is given in the table. Total cost of parts other than electrodes is thirteen dollars if the milliammeter itself costs four dollars. Such an inexpensive milliammeter has an internal resistance of about 1000 ohms. The value of resistor R in series with the milliammeter is chosen such that its resistance plus that of the amp fuse (about 40 ohms) plus that of the milliammeter itself totals 1200 ohms. The presence of such a high resistance in series with the conductance cell introduces a nonlinearity of 20%. If the rnilliammeter and the resistors are each accurate to 50/0,conductance measurements are accurate to about 30%, apart from any errors due to polarization at the electrodes. Use of precision resistors and a more accurate milliammeter with low internal resistance would reduce the nonpolarization error to 50j0,hut would increase the cost to twenty-five dollars. This change would require that each shunting resistance he reduced by the same factor as is the meter resistance.
1.8
Figure 1.
Parts List for Conductance Meter 1 Chassi~,7 X 5 X 2 inellel 1 Purl, lillfton momentar)- switch. sin& pole. normally open I Rotary rrvitcl~, single section. sine1c ,,ole, shorting. 11 poritions 2 Fuse rnnuntings, 8 AG 1 Fuae, 8 .\G. 2 am" 1 Fuse, 8 .A
