charge rather than a negative one. In any case, clockwise or counterclockwise, positive or negative, the principle holds that the changes in

electric potential around a complete circuit loop sum to zero.

This section can be used to illustrate a more general principle: As you traverse a circuit, you can add the potential differences across the

components. For instance, if you traversed two resistors, the potential difference across the two resistors would equal the sum of each

resistor’s potential difference.

27.4 - Measuring current and potential difference

Ammeter: A device that measures current in a

wire.

Voltmeter: A device that measures the potential

difference between two points.

Ammeters and voltmeters are common tools for analyzing circuits. Both are shown

above.

Ammeters measure current. An ammeter is inserted in the circuit, as shown in Concept

2, so that all the current in that part of the circuit flows through it. Because current flows

through them, ammeters are built to have extremely low resistance in order to minimize

their impact on the circuit. An ideal ammeter would have no resistance.

A voltmeter measures the potential difference (voltage) between two points in a circuit.

In Concept 3, the potential difference across the two terminals of a battery is being

measured. To use the voltmeter, a lead is placed on each side of the battery.

Voltmeters are also designed to have minimal effects on a circuit. In the case of the

voltmeter, this means they are designed to have a high resistance, so little current flows

through them, and the rest continues to flow through the circuit component being

measured.

Ammeters and voltmeters often are combined into one instrument called a multimeter. A

multimeter may also contain a device for measuring resistance, called an ohmmeter.

Ammeters and voltmeters

Ammeter

Measures current

Voltmeter

Measures potential difference

27.5 - Series wiring

Series wiring: Circuit wiring in which the

components are placed one after another. All of

the current flows through each component.

The diagram to the right shows an example of a series circuit. The same amount of

current passes through the battery and through each light bulb. The circuit has no

branches; there are no places where the current can split to follow another path. The

light bulbs and the battery are said to be connected in series.

Place an ammeter anywhere in this circuit and you will measure the same value for the

current. The same amount of current passes through the battery, the first light bulb, the

second light bulb and the wires that connect these components.

If the current in a series circuit is interrupted anywhere, it is interrupted everywhere.

Because there are no alternative routes, if the circuit is broken at any point, then there

Series wiring

Current has one path through

components

(^492) Copyright 2000-2007 Kinetic Books Co. Chapter 27