Lesson 3/Learning Event 1
Another example of daily measurement is the water and electricity supplied to our homes. These
measurements are made with meters that measure how much you use (volume or flowmeters).
Water and electricity bills are based on these meter readings. You have probably seen at least one
of these meters as the electricity meter should be mounted on your house in plain view. These
flowmeters, which are read by the "meter man," are only one of the many types of measuring
devices needed to show service personnel how a system is operating. Both water and electricity
systems work best in a certain pressure range. Both systems, therefore, include pressure-measuring
devices at various points along the line.
Automotive electrical systems also require various measurements to indicate how they are operating.
The automotive electrical system actually resembles the water system in your home in many ways.
You know that voltage is measured with a voltmeter. Let's compare the flow of voltage with a
water system.
Have you ever opened the valve or spigot at your bathroom sink and the water flowed very slowly?
The next time you opened the valve a few hours later the waterflow was back to normal. The
problem was probably caused by a pressure loss in the water system. If you could have looked at a
pressure gage on the water system, you would have found the pressure was lower than normal
when the water flowed too slowly. This tells you that the pressure in a water system affects the
flow. The same is true of an automotive electrical system. When the pressure (voltage) drops, it
causes the flow (current) to drop also and makes electrical components work improperly. For
example, electric motors run too slow, solenoids or electromagnets do not have enough strength to
operate, and lights and lamps do not burn brightly.
Let's compare a simple electrical system to a simple water system and see how the voltmeter
measures pressure. Notice that the electrical system, shown in Figure 20 contains a battery on the
left. The battery is connected to the load on the right. The load could be any electrical
component, such as a light bulb, electromagnet, or an electric motor. The current flows from the
battery to the load, then through the load and back to the battery. Like the water system, the
electrical pressure gage (voltmeter) must sample the pressure or voltage in this circuit.
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