Journeyman Electrician: The Basics of Measurements

Measurements

Continuity and Resistance Measurements
Simple continuity, or the absence of it, can be detected with a test lamp.  If it lights then there is continuity, if not then there is not.  This can be useful in detecting short or open circuits in condensers and coils.
A better indication is provided by the Ohmmeter.  When using an Ohmmeter, be sure to disconnect or remove any external voltages from the circuit being tested; otherwise a number of bad things can occur.  Incorrect readings will result if external voltages are present, and in the worst case damage to the meter may result.
When the needle of a volt-ohm meter will not align with the zero mark for resistance measurements, but there is no problem when it is used for other functions, the battery that is used for the Ohmmeter function is probably weak

Electrical Current Measurements
When connecting the ammeter, the operator must open the circuit under test and connect the meter in series to complete the circuit. 
Care must be taken not to damage the circuit or the meter.
Because of the high voltage that can be developed from the inductance of transformers, when disconnecting an ammeter from an in service transformer the secondary terminals must be shorted.  Before removing an ammeter from the current transformer, its secondary must be short circuited to avoid damage from the induced voltages as mentioned above.
To increase the range of an AC ammeter a current transformer is sometimes used.

Voltage Measurements
Voltage measurements can usually be made without affecting the circuits under test, from almost any point to any other.  Voltmeters are connected across any source or load to read the voltage or IR drop.  

Allow the test circuit to remain as is and connect the meter in parallel across two points.

There are some precautions that must be taken with voltage measurements.  

When measuring an unknown voltage, always start with highest range on the meter.  When measuring DC voltages with analog meters, reversed polarity can damage the meter.  Be prepared to quickly check the meter for the proper response and disconnect the leads if the polarity is not as expected.
 

Measuring Illumination
Light intensity is measured in foot candles, equal to the intensity of a standard candle at a distance of one foot.  The metric unit is the lux which is equal to about a 10 foot candle.
Illumination, or luminous flux, is the measure of the total light emitted from a device and is measured in lumens.  The light output is limited to the visible portion of the light spectrum.  The measured amount of illumination does not include infrared or ultraviolet light and is adjusted to compensate for the varying sensitivity to different types of light of the human eye.
The foot-candle is equal to one lumen per square foot.  A 100 Watt incandescent light bulb produces an illumination of about 1700 lumens.  By comparison, a fluorescent lamp with the same power consumption may emit 5000 lumens.
The best illumination is provided by numerous lights evenly spaced.

Measuring and Marking Devices
The most common physical measuring device an electrician uses is a tape measure. Steel measuring tapes are undesirable because of possible entanglement in machinery and the short circuit and shock hazard if they are used around live circuits.  A plastic or wooden ruler or tape should be used to measure wherever there is live equipment.
To determine whether something is vertical, a spirit level is used.  A plumb bob is used to mark a point on the floor directly below a point on the ceiling.  To lay out a straight line, use a chalk line.
A dial indicator is used to check motor shaft alignment. 
The instrument used to measure the diameter of a wire, conduit, drill bits, or other small objects to thousandths of an inch is a micrometer.