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1EXPERIMENT #3: OHM'S LAW EXPERIMENT
PRELAB INSTRUCTIONS
PART I
 OHM'S LAW
When George Simon Ohm investigated the relationship between voltage
and current, he found that generally
, the ratio of supply voltage (E) to the
resulting current (I) was constant.
He meant that if the applied EMF were
doubled, the current would also be doubled.
Stated mathematically:
E / I
produces a constant ratio
The constant of proportionality is called the resistance (R) of the circuit.
So Ohm's Law states that
generally R is constant.
E (in Volts) / I (in Amperes or Amps) = R (in Ohms)
Use the memory aid below, called the Ohm’s Law Triangle, for calculating any of the three
values when two other values are known:
E/I=R
E/R=I
IxR=E
The resistance of most electrical conductors is constant and when R is constant the circuit
(resistor) is said to be linear.
When R changes with changes in applied current, the resistance is
called nonlinear.
Example:
A voltage of 5 kV is applied to a resistor and the resultant current is found to be
2mA, what is the resistor?
R = E / I = 5,000V / 0.002
Α
= 2,500,000
Ω
= 2.5M
Ω
EXPERIMENT #3: OHM'S LAW EXPERIMENT
Ohm's Law
Triangle
E vs. I Characteristic
of a Linear Resistance
E vs. I Characteristic
of a NonLinear Resistance
23
E
R
I
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View Full DocumentPART II
 MULTIRANGE METERS  CHOICE OF RANGE
Philips, Hioki, Simpson, and Micronta multimeters are examples of analogue meters. In such a
meter the magnitude of the deflection of the pointer represents a definite magnitude of, say
voltage or current.
A selector switch enables the user to choose whether the meter will read
voltage or current, and also the range of measurement possible, e.g. 0 to 3 V or 0 to 300 V.
Experience has shown that when you are uncertain about how large the voltage or current to be
measured may be, you should choose the highest range then systematically switch down ranges
until you get a reading in the upper half of the scale.
Analogue meters tend to be less accurate
in the lower part of the scale for reasons you will learn in your Measurements Course.
While it
may not always be possible, do use the range that gives you a reading in the upper part of the
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 Fall '08
 Usik
 Gate, Volt

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