A Major Experiment for Teaching MeaningfulElectricity Fundamentals

R. E. ShowersEast Hi^h School, Green Bay, Wisconsin

We all search for learning experiences which students enjoy andwhich we feel do a genuine job of teaching. Student comments on thefollowing electricity experiment have been gratifying. The low costof the basic equipment; the availability of components; ease of as-sembly; and the practicality of being drawn from the electronics fieldshould enhance the experiment in the teacher’s eyes. Most of all, thisexperiment can be a culminating laboratory effort after study ofOhm/s law and KirchofFs Laws. It can be the first experiment donein electricity and the students will have a genuine sense of accom-plishment when they complete it. The equipment is extremely flexibleso that better students can proceed to relatively complex under-standings with ease.

BACKGROUND

A survey of the basic electricity experiments presented in labora-tory manuals stimulated a search for some kind of a laboratory ex-perience that would point up Ohm’s law and KirchofTs Laws morerealistically. To me, experiments with nichrome wire resistors and re-sistance boxes were inadequate.When the available equipment for the study of electricity in our

school laboratory was examined, I found only ammeters with amonstrous 10 amp range and D.C. voltmeters with ranges of 15volts. I was bequeathed an antiquated generator system with a bat-tery bank which was no longer functional.

PLAN

During the next three-year period specific equipment was acquiredto implement the new program. Four rectifiers which would produceup to 8 volts D.C. were purchased. This was less expensive than re-juvenating the D.C. system of the laboratory. Four 10 watt 50 ohmwire wound potentiometers, twenty binding posts, and sixteen as-sorted carbon resistors of one watt rating were purchased. During theperiod four dual range milliammeters were purchased. These hadranges from 0-15 mils and 0-150 mils. We also found on the surplusmarket some excellent multirange voltmeters. These had ranges from1.5 to 15 volts in three or four steps. We completed the equipment de-mands with one moderately priced multirange volt ohm multimeter.The heart of the experiment is a resistor board. Four pieces of

4^X6^ plywood were cut from y stock. Five binding posts were

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affixed; one in the center and one in each corner of the board. Suchan arrangement allows the insertion of four resistors diagonally with acommon center terminal. Two identical resistors were used as well asone low one and one of higher resistance. The four resistors were sold-ered to a common central lug and had a lug soldered on each of theother terminals so that the entire set-up was electrically sound andonly binding post connections would be subject to a friction fit.

The Sefc-Up

STUDENT PREPARATION

Before we undertake the laboratory work the class studies Ohm’slaw with emphasis on units and problem solving. Currents are intro-duced as milliamperes. When meters are studied emphasis is placedon the accuracy obtained near full scale and the errors introduced byreading less than half scale. When we take up KirchofFs laws the factthat meters pass current received attention. Meters thus becomeassociated with voltage drops and the meaning of internal resistanceof a meter becomes more significant. The application of voltmetersin circuits is referred to as a ^parallel^ procedure, and hence the cur-rent flowing through a voltmeter is part of the current indicated in

’Rectirttt^^UTVvT

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-fi-

Res »ST 0*1 "Bo^^p

The Set-Up from above

the total circuit. The ohmmeter is studied as a self-contained Ohm/slaw circuit carrying its own source of EMF.

PROCEDURE

Students are cautioned to use the high scale of the milliammeter foreach set-up. With only the switch in the main circuit closed the po-tentiometer is manipulated to give the highest ^on scale^ reading ofthe meter. If the next lower scale can be used, the range of the meteris lowered and once again the pot is adjusted. Assuming this is ac-complished and the scale reading is adequately high it is recorded,

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Potentiometer Mounting

and the voltmeter is momentarily closed for a reading. If the milliam-meter goes off scale it must be put on the next higher range. Thereading taken at the lowest ^on scale" range of the voltmeter is re-corded as the V of the circuit. The I and V values are used to computeR. Leads are now disconnected from from the board and the ohmeterapplied across resistor I. The reading is compared with the calculatedvalue. Since the resistors are standard radio types, the color code isread including the tolerance value. The color code value is recorded.Per cent error between the color code value and the experimentalvalue is calculated. It should fall within the tolerance factor.The same procedure is followed for resistors II, III, and IV and for

the series circuits developed by using I and II, II and III, III and IV,and IV and I.The students are now required to submit diagrams to show how V

and 7, for resistors I and II in parallel; II and III in parallel; III andIV in parallel; IV and I in parallel; and I and II in parallel with III inseries with the parallel may be obtained.

Experimental values for each resistance and the ohmmeter valuesare recorded. The theoretical value is obtained by doing a parallelproblem using the color code values. Once again per cent error iscalculated using the experimental value and the color code value.

Teaching Electricity 71

The better students are encouraged to collect data on the multipleseries and parallel circuits to demonstrate KirchofTs laws. Currentreadings on the parallel set ups usually are very good but the volt-age checks are never as convincing because the readings seldom areconsistently high on the scales. The potentiometer control ofcurrent each time serves to increase the accuracy of the currentreadings.

This experiment has been a satisfactory learning device for a num-ber of years. During this time no meters have been destroyed andthere have been frequent expressions on the part of students thatOhm’s law and KirchofTs laws were more fully appreciated afterworking with the experiment. Budgetwise the equipment is notcostly and it is easily replaceable from readily available parts. Per-haps it is time that we all review the appropriateness, the teachingefficiency, and the cost of our elementary electricity experiments.

Potentiometer Mounting

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Resistor Board

Resistor Board