NUE 052 Units of Measurement & Meters

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NUE 052 - Units of Measurement & Meters

The four basic units of electrical measurement are Volts, Amps, Ohms and Watts.

Volts is electrical pressure - the potential difference measured between 2 points in a circuit

Amps is electrical current - the (conventional) flow of current in a circuit from positive to negative

Ohms is electrical resistance - the opposition to current flow in a circuit

Watts is electrical power - the electrical energy in a circuit

Click here to see Ohms law using Volts, Amps, Ohms and Watts

To measure electrical units we need to use an instrument called a meter.

Meters come in all shapes and sizes, each for a different purpose. We can have special purpose meters that measure just one type of unit such as watts or an instrument called a multimeter which is capable of measuring volts, amps and ohms.

Even though the multimeter can measure three electrical units, the meter must be connected differently in the circuit.

To measure Volts - connect the meter in parallel with the circuit under measurement while connected to the supply.

To measure Amps - connect the meter in series with the circuit under measurement while connected to the supply.

To measure Ohms - connect the meter in parallel with the circuit under measurement with the supply disconnected.

Click here to learn more about meters Click here to learn more about Cathode Ray Oscilloscopes (CRO)

Multiples and sub-multiples

Electrical measurements can be from thousands of amps to millionths of an amp.

Terms used for these measurements 'kilo' and 'milli' are called multiples and sub-multiples.

A few examples are listed in the following table:

Term	Symbol	Multiplier	Example
mega	M	1,000,000	107MHz = 107,000,000 hertz
kilo	k	1000	5kV = 5000 volts
milli	m	0.001	6mA = 0.006 amps
micro	u	0.000001	2uA = 0.000002 amps

Conversion Examples

1. A resistance reading is 4.7 Megohms. How many ohms does this represent?

Answer:

Mega is one million so multiply 4.7 by 1,000,000 by moving the decimal point six places to the right 4,700,000 ohms

2. There is 2.45 mA flowing in a circuit. How many amps is this?

Answer:

Milli is one thousand so divide 2.45 by 1,000 by moving the decimal point 3 places to the left 0.00245 amps

Scientific Notation

Scientific Notation is a way of expressing multipliers (see previous table) in mathematical form. Each multiplier is treated as a power of 10. Following are examples scientific notation using the examples from the previous table:

Term	Symbol	Multiplier	Scientific Notation	Example
mega	M	1,000,000	106	107MHz = 107,000,000 hertz
kilo	k	1000	103	5kV = 5000 volts
milli	m	0.001	10-3	6mA = 0.006 amps
micro	u	0.000001	10-6	2uA = 0.000002 amps

Conversion Examples

1. Convert 10kV to volts

Answer:

10kV	=	10 x 103 V	(k = 103)
	=	100 x 102 V	(1 decimal place right)
	=	1000 x 101 V	(2 decimal places right)
	=	10000 x 100 V	(3 decimal places to right)
10kV	=	10,000 V

2. Convert 0.10mA to microamps (uA)

Answer:

0.10mA	=	0.10 x 10-3 A	(m = 10-3)
	=	1.0 x 10-4 A	(1 decimal place right)
	=	10 x 10-5 A	(2 decimal places right)
	=	100 x 10-6 A	(3 decimal places to right)
0.10mA	=	100 uA	(u = 10-6)

Practice Tests	Practice tests are available on the Practice Test page
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