Consider the five measurement approaches presented above, and identify the one t

Question

Consider the five measurement approaches presented above, and identify the one that seems most precise. (That's not, probably, the one that comes closest to the labeled value.) You base R_1 by this technique, now measure R_3, the series combination of R_1 and R_2, and the parallel combination of R_1 and R_2. How well do your data support the formulas for series and parallel combinations? Derive the ohmmeter equation for the ohmmeter circuit shown in Fig. 6. Plot a theoretical curve of R_X (resistance to be measured) as a function of ammeter current, using the constants of the meter to be used and a dry cell voltage of 1.5 volts. Wire up the ohmmeter circuit, using your ammeter, the resistance, box as the variable resistor (setting it to the value predicted in theory (see the Prelab) with aid of the Wheatstone bridge), and a flashlight battery, and adjust it properly. Measure the several mounted calibrating resistors with the Wheatstone bridge, and using these resistors as standards, calibrate the ohmmeter by plotting the experimental curve of R_X (R from the bridge) versus ammeter current in the ohmmeter circuit, on the same plot as the theoretical curve. Copy the face of the ammeter, with its actual current markings, and add markings to your copy so that the meter would indicate resistance values. Explain in detail the relative merits of the two ammeter-voltmeter methods. In part 2, why were the voltmeters exchanged and a second set of readings taken? Why is it desirable to obtain a balance with the sliding contact in the middle third of the R_1 - R_2 resistance when using the Wheatstone bridge? Before using an ohmmeter in a radio circuit (or anywhere else) what should one do to the power in the radio circuit? What can one determine about a capacitor by using an ohmmeter? What adjustment does one have readily available to compensate for aging dry cells (and other variations) in ohmmeter circuits? Explain briefly how this compensation works.

Explanation / Answer

1. Explain in detail the relative merits of two ammeter - voltmeter method.

ans :

2. ........

ans :

In first set, the ammeter measures the true value of the current through the resistance but the voltmeter does not measure the true voltage across the resistance. The voltmeter indicates the sum of the voltages across the ammeter and the measured resistance.

In second set, the voltmeter measures true value of voltage but the ammeter measures the sum of currents through the resistance and the voltmeter.

3. ............

ans:

The slide wire form of the Wheatstone bridge is shown. The line ad represents a wire and C is a contact key that slides along the wire to divide the wire into L1 and L2. Since the resistance of the wire segments are proportional to the lengths of the wire, then

R2/R1 =L2/L1

we know that, Rx =(R2/R1)Rs

Rx=(L2/L1)Rs

5. .................

ans :

We take the ohmmeter and place the probes across the leads of the capacitor. The orientation doesn't matter, because resistance isn't polarized.

If we read a very low resistance (near 0) across the capacitor, we know the capacitor is defective. It is reading as if there is a short circuit across it.

If we read a very high resistance across the capacitor (several M), this is a sign that the capacitor likely is defective as well. It is reading as if there is an open circuit across the capacitor.

the basic principle are, the capacitor is to conduct A.C. only.

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