An Introduction To Electrical Measurements
Introduction
Measuring Voltage
Measuring Current
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Introduction

Electrical measurements often come down to either measuring current or measuring voltage.  Even if you are measuring frequency, you will be measuring the frequency of a current signal or a voltage signal and you will need to know how to measure either voltage or current.  In this short lesson, we will examine those two measurements - starting with measuring voltage.  However, first we should note a few common characteristics of the meters you use for those measurements.

Many times you will use a digital multimeter - a DMM - to measure either voltage or current.  Actually, a DMM will also usually measure frequency (of a voltage signal) and resistance.  You should note the following about typical DMMs.

• Polarity is important.  Usually the terminals of the DMM will be coded to indicate polarity.  Often that polarity is indicated by a red terminal (positive) and a black terminal (negative).  In other cases, the polarity could be indicated by printed notes on the terminals.
• Often one of the terminals on the DMM may be connected to the ground.  That would normally be the black terminal, or it may be indicated with a ground symbol.
With that in mind, let's get on to measuring voltage.
Measuring Voltage

Voltage is one of the most common quantities measured.  That's because many other variables - like temperature, for example - are measured by generating a voltage with a sensor.  So, even if you want to measure temperature you might end up having to measure a voltage and convert that reading into the temperature reading you wanted.

Voltage is measured with a voltmeter.  However, digital multimeters (DMMs) - which can function as voltmeters - often have considerably more capability and can measure current, resistance and frequency.  And, there are other instruments - like oscilloscopes - that measure voltage and should be thought of as voltmeters.  No matter what the instrument is, if it measures voltage you have to treat the instrument as a voltmeter.

When you measure voltage you have to remember that voltage is an across variable.  When you measure voltage you have to connect the voltmeter to the two points in a circuit where you want to measure voltage.  Here is a circuit with a voltmeter connected to measure the voltage across element #4.

• Notice that the voltmeter measures the voltage across element #4, +V4. (And, the plus sign is important.  Remember the polarity issue.)
• Notice the polarity definitions for V4, and notice how the red terminal is connected to the "+" end of element #4.  If you reversed the leads, by connecting the red lead to the "-" terminal on element #4 and the black lead to the "+" end of element #4, you would be measuring -V4.
And, remember this as well.
• When you measure voltage, the voltmeter should not disturb the circuit where you are attempting to measure the voltage.  In the circuit above, that disturbance is the current drawn by the voltmeter.  You want that current to be as close to zero as it can possibly be.  That means that you need to have the resistance of the voltmeter as large as possible.  There's more discussion of that effect in the lesson on measuring voltage.  Ideally, the resistance of a voltmeter would be infinite.
There are numerous different instruments, and  we have a separate lesson just on measuring voltage that discusses some of those instrument, and you can get to that lesson by clicking here.
Measuring Current

Current is measured with an ammeter.  While voltage is a more common measurement, it is often necessary to measure current.  When measuring current, it is important to remember that current is a flow variable.  Current flows through electrical elements, and if you want to measure current you have to get it to flow through the ammeter.  Here's the same circuit we used in the example above.  Consider what we would have to do to measure the current flowing through element #4.

If we want to measure the current through element #4, we have to get that current to flow through the ammeter.  Here's a way to insert an ammeter into the circuit to measure that current.

However, this doesn't give the whole picture.  Remember that polarity is important.  In the circuit the  polarity for the voltage across element #4 is defined, but the current polarity is not defined.  In the diagram below, we have defined the direction of that current, and given it an algebraic name, Im.

As with the voltmeter, you need to pay attention to the polarity, and you also want to remember this.

• When you measure current, the ammeter should not disturb the circuit where you are attempting to measure the current.  In the circuit above, that disturbance is the voltage across the ammeter.  You want that voltage to be as close to zero as it can possibly be.  That means that you need to have the resistance of the voltmeter as small as possible.  Ideally, the resistance of an ammeter would be zero.

Links to Related Lessons