A DEMONSTRATION POTENTIOMETER PAUL BENDER University of Wisconsin, Madison, Wisconsin
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potentiometer is, of course, one of the basic instruments employed by the student in the undergraduate physical chemistry laboratory. It becomes a real problem for the laboratory instructor to see that the necessary apparatus is provided in a form which permits efficient use of laboratory time and also permits a full understanding of the operation of the instrument from the practical and theoretical points of view. The requirements of ruggedness and accuracy in the measuring unit itself are well met by the standard units supplied under the name "student potentiometer" by Leeds and Northrup, Ruhicon, etc. The manufacturers suggest that these units are particularly desirable from the pedagogical point of view because they permit the student to assemble for himself the auxiliary apparatus external to the actual decade and slidewire combination, thus retaining the teaching advantages of the older ouen slidewire unit whiie obtaining- greater accuracy. For several reasons, however, it is preferable to have for undergraduate physical chemistry laboratory work a complete and compact unit assembled and ready for the student, who is primarily concerned here with the use of the potentiometer as a tool, in contrast to his
Even if the student possesses a reasonably sound understanding of the basic potentiometer principle, it may prove difficult for h i to understand its application in the particular instrument he is using. This has often been observed with the student-type potentiometer, whose internal wiring is not normally open for inspection and whose wiring diagram is complicated by
the presence of the range-shifting resistors and the end coils used in its conductivity-measurement application. We propose as a solution to this .problem the construction of the demonstration potentiometer illustrated in Figure 2, which is essentially a duplicate of the regular lahoratory unit assembled in a transparent plastic case with the wiring arranged to facilitate the tracing of the circuit. The case is constructed of Lucite sheet; the front and back panels are of one-quarter-inch and the frame of one-half-inch stock. The panels are set flush with the frame on milled shoulders. and the assemblv is completed with nickel-plated 'brass screws. ~ i decade e Figure 1. Student Potentiornet.= Asmrably and slidewire only from a Leeds and Northrup student potentiometer are included in the instrument. The study of the potentiometric method itself in the physics end coils previously mentioned are not pertinent here, lahoratory. Such an arrangement offers the advantage and the omission of the range-shifting resistors is jusof affordmg a marked saving of the students' time, of tified on the grounds of the resulting gain in clarity and permitting more students to be senred effectively with a of the f a d that our laboratory potentiometers are used given number of potentiometers, and of simplifying on the 0 to 1.6-volt range only. maintenance problems while prolonging the life of the The three resistors employed to adjust the operating equipment. A typical installation of this kind, used current are, respectively, ZOO-, 20-, and 2-ohm Centralab successfully a t the University of Wisconsin for several V series wire-wound radiohms. The toggle switch visible below the slidewire dial is in series with the two No. years, is shown in Figure 1. 195
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6 dry cells used as the working battery. The tap keys in the galvanometer circuit are Centralab momentary push switches; three rather than two are employed to impress more forcefully upon the student the necessity of taking proper precautions in making preliminary adjustments, and they close the circuit, respectively, through series resistors of ten thousand, one thousand, and zero ohms. An H & H, D.P.D.T., rotary switch is used as the selector switch; the leads for connection to the unknown e. m. f. source pass through the lower left comer of the front panel. The galvanometers employed with the student potentiometers are of the Rubicon Spotlite type, which offer a comparatively high sensitivity in a rugged lamp and scale instnunent. For practical reasons it has been found best to have the galvanometer cord (which incorporates the required stepdown transformer) plug into an a,-c, outlet at the rear of the potentiometer case. The separate lime required for connection to the 110-v.
JOURNAL OF CHEMICAL EDUCATION
main is provided with a S.P.S.T. H & H rotary switch and a pilot light. The galvanometer damping resistance is permanently connected in parallel with the galvanometer coil; the resulting decrease in sensitivity is negligible since the damping resistance is about six times the coil resistance. If it is desired to have the damping resistance momentarily cut out when the galvanometer circuit is closed, the use of H & H two-circuit push switches is recommended. These and all other auxiliary parts referred to are available as standard items from radio parts jobbers. ACKNOWLEDGMENT
The apparatus described in this article was constmcted in the departmental mechanician's shop. The author wishes to express his appreciation to Mr. Lloyd E. Lincoln who was particularly concerned with this project.
