HOW A VOLTMETER WORK

You know that an ammeter measures the rate at which electric charges move

trough a material, and that the rate of current flow trough given material varies

directly with the potential difference (or "voltage") across it. That is to say the

greater the voltage, the greater (other things being equal) will be the current flow.

A meter called a voltmeter measures voltage. It consists of an ammeter placed

series with a special piece of material, which limits current flow. Measuring the

current that flows in meter circuit makes the measurement of voltage. The special

piece of material is called a multiplier resistor. For a given ammeter and given

multiplier resistor, large current will flow when the voltage is high, and small

current will flow when the voltage is low. The meter scale is calibrated in volts,

and read directly. It is the multiplier resistor which determines the scale range (as

did that shunt in the ammeter)

It is always desirable to keep current flow in a voltage measuring circuit as low as

possible, lest the act of connecting the meter was not present. For this reason,

the meter used is always a millimeter or micrometer-never an ammeter.

The multiplier resistor will be built into most of voltmeter you will use, so you can

measure voltage by making very simple connection. Whenever the (+) meter

terminal is connected to the (+) terminal of voltage source, and the (-) meter

terminal to the (-) terminal of the voltage source; with nothing else connected in

series, the meter will reward voltage directly.

Whenever you are using a voltmeter, take a great care to observe correct meter

polarity, and to choose a meter having a maximum scale range greater than

maximum voltage you expect to read.

VAN VALKENBURGH, NOOGER & NEVILLE, Inc.

Basic Electricity

A. Answer the question briefly.

1. What is the function of ammeter?

2. How is the current flow when the voltage is weak?

3. Why does multiplier resistor placed in a voltmeter?

4. Why do we always use a millimeter or micrometer as the meter?

5. What should we choose in a meter when we want to measure a high voltage?

B. Find the synonym of the words from the box.

1. Finding the size, extend volume, degree, etc.

2. Regular series of marks for thee purpose of measuring
3. Part or bit of something

4. Place for something come from.

5. Point or line that may not or cannot be passed

III. Decide which statement is true or false.

1. The voltage will be high if the current is large.

2. We have to choose a meter having maximum range lower than the voltage we

want to measure.

3. We do not need a multiplier resistor in a volt meter to limit the current flow.

4. The multiplier resistor determines the scale range.

5. We always use ammeter as the meter to measure the voltage.
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HOW A VOLTMETER WORK