Life and death - HumanBodyResistance;...

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Practical Perspective: Human Body Resistance; Life and Death in the World of Electricity © 2010 Alexander Ganago Page 1 of 12 File: Life and death 2.6. Practical perspective: How to avoid blunders in measurements of electric resistance? Now we can explain the resistance measurements discussed in the beginning of this chapter and point out a typical blunder. First of all, let us see how an ohmmeter works. As you already learned, the resistance R of a circuit component can be calculated as the ratio of the voltage V Probe across and the current I Probe through this component. This is exactly what happens in an old‐ fashioned ohmmeter: it applies a small voltage (such as from a 1.5‐V battery) across the terminals, to which you connect the resistor whose resistance you want to measure, and measures the current through these terminals with the built‐in ammeter; the unknown resistance is obtained as their ratio R = V Probe I Probe (see Figure 2‐33). Figure 2‐33. An ohmmeter, which is built the old way, applies a small voltage V Probe to the resistor whose resistance you are measuring and measures the current I Probe through it. The equivalent resistance R is calculated as the ratio (V Probe /I Probe ). Note that only the resistor R is connected to the instrument’s terminals, which are shown as small circles.
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Practical Perspective: Human Body Resistance; Life and Death in the World of Electricity © 2010 Alexander Ganago Page 2 of 12 File: Life and death You have also learned that the unknown resistance can be calculated from formulas for voltage division (see Application #1 in section 2.3.5, the diagram in Figure 2‐20 and the equations derived for this circuit). This idea is used in a more modern ohmmeter that applies V Probe ~ 1 V to a series combination of the unknown resistance R, which you connect to its terminals, and the internal reference resistance R Ref ; the built‐in voltmeter measures the voltage across R Ref ; the unknown resistance is calculated as we did in section 2.3.5 and displayed for the user (see Figure 2‐34). Figure 2‐34. A more modern ohmmeter applies a small voltage V Probe to a series combination of the resistor whose resistance you are measuring and the reference resistor R Ref , measures the voltage across R Ref , and calculates the unknown resistance R from voltage division as we did in section 2.4.4. Again, only the resistor R is connected to the instrument’s terminals, which are shown as small circles. The correct way to measure resistance is sketched in Figure 2‐1 at the beginning of this chapter. It applies to any type of ohmmeter. The essence is that only the resistor whose resistance you need to measure should be connected to the instrument’s terminals. The circuit involves only the resistor R to be measured and the internal circuitry of the ohmmeter (Figure 2‐33 or 2‐34). The wrong way to measure resistance is sketched in Figure 2‐ 4 and repeated with explanations in Figure 2‐35: distortion takes place when the user touches the connectors of the resistor during measurement.
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