"Electric Devices and Meters"
Physics PHA202 PT2E
Santos, Nickerson Jeff F. February 27, 2002
Experiment No. 7
Electric Devices And Meters
To familiarize the students with the different types of electric devices and meters.
In order to perform experiments dealing with electric circuits, you must become familiar first with the devices and meters needed for such experiment. Some of the most common devices and meters to be used is described below.
1) RESISTORS Resistors are devices used to give a desired value of resistance. They are inserted into a circuit in order to lower the current of to produce a desired voltage drop. There are two kinds of resistors: a) the Fix Resistor and, b) the Variable Resistor. The fix resistor has a specific value of resistance which cannot be changed, while the variable resistor may have a any value of resistance within a specified range. Resistors have two important characteristics: a) its resistance in Ohms and, b) its wattage rating. Resistors are available in a wide variety of resistance values ranging from less than 1 ohm to mega ohms (MΩ). The power rating may be as high as 100 watts or may be lower than 1 watt. High-wattage resistors are usually of the wire wound type while low-wattage resistors are the carbon-composition type. The latter type is the one that we ordinarily see in which the resistance value is indicated by color codes. It is a fix resistor. For the variable resistor, it may be the plug type, the dial type or the switch type.
2) POTENTIOMETER This is a variable resistor with three terminals. The two end terminals gives its fix maximum resistance while the middle terminal and an end terminal makes it a variable resistor.
3) RHEOSTAT This is a two terminal variable resistor that is used to vary the current in the circuit. Generally, rheostats are high-wattage wire wound resistors.
4) SWITCHES Switches are devices used to make or break circuits. They are classified by the number of poles and the number of throws. For example, the single-pole single-throw (SPST) switch with only one pole and can be thrown only in one position.
5) BATTERY / DRY CELL Batteries serves as a source of electricity of the direct current (DC) type. It is usually made up of cells connected in series to give a desired value of voltage. It has two terminals, a positive and a negative terminal.
6) TRANSFORMER A transformer is an electrical device capable of changing a voltage at one level to another whether higher or lower. It is commonly used to transform a 220 volts AC to 6 and 12 volts AC.
7) VARIABLE AUTOTRANSFORMER (VARIAC) This is a
special type of transformer capable of changing a voltage at one level to another level which may be varied from a specified minimum value to a specified maximum value. For example, the input voltage is220 volts, the output voltage may range from 0 to 220 volts. This device is very important in situations where different values of voltage is needed.
8) RECTIFIER This is an electronic device used to change AC voltage to DC voltage through the action of diodes. A variac-rectifier combination is capable of giving a variable DC output.
9) AMMETERS These are metering devices for the measurement of electric current. It may be of the DC or AC type. Furthermore, the term ammeter refers to meters capable of measuring current in amperes. For low values of current, there are the milliammeter and the microammeter.
10) VOLTMETERS These are metering devices for the measurement of voltages. Like ammeters, it may be of the DC or the AC type.
11) OHMMETER This is an instrument for the measurement of electric resistance. This function is usually part of a multimeter arrangement like the VOM which will be discussed next.
12) VOLT-OHM-MILLIAMMETER (VOM) This is a popular