Page 509 - Canadian BC Science 9

Science Skill 11
Using Electric Circuit Symbols and Meters
Circuit Diagram Symbols
The Terminals of a Meter
All meters have two terminals (connecting points) that you connect to the circuit. The negative terminal (􏰂) is black. The positive terminal (􏰃) is red. In a circuit, conventional current is defined as flowing from positive to negative. This means that current leaves the positive side of the battery or power supply and returns to the negative. In order not to damage the meter, you must take care to connect the meters so that the positive (red) terminal of the meter is connected to the positive side of the power source. That is, if you trace the current leaving the source it should enter the meter through its positive (􏰃) terminal. The negative (􏰂) terminal of the meter is always connected to the negative side of the source. The rule is “positive to positive, and negative to negative.”
Connecting an Ammeter
An ammeter is used to measure the electric current in a circuit. Electric current is the amount of charge passing a given point per second. To measure the current at a given location in an electric circuit, the ammeter must be connected so that all the current is allowed to pass through the ammeter. To do this, you must disconnect the circuit at the location where you wish to measure the current. Then insert the ammeter so that current leaving the power source enters the positive (red) terminal of the ammeter and leaves from the negative (black) terminal.
B. Analogue meters have a needle pointing to different scales.
􏰃 􏰂
􏰃􏰂
conducting wire voltmeter
cell
battery bulb
open switch
closed switch resistor ammeter
Using Meters to Measure Voltage and Current
Types of Meters
The meters you use in your classroom are either analogue meters or digital meters. Analogue meters are meters that have a needle pointing to a dial. Digital meters display measured values directly as numbers, similar to how a digital watch displays the time directly.
A. Digital meters display the numerical values directly.
Science Skill 11 • MHR 491