To understand A.C. current and voltage,  you must first understand D.C. current and voltage,   a  D.C. current is a current that flows from a positive terminal to a negative terminal,  such as in a battery or D.C. power supply,  in this case all points nearer the positive terminal will measure more positive than points near the negative terminal.

    In A.C. current and voltage,   the situation is the same except that the positive and negative voltages change ends at a rapid rate,    this means that one voltage cannot be more positive or negative than another,  since one moment  it is more positive and the next more negative.

    To measure A.C. voltage and current we must ignore whether the voltage is positive or negative and only consider the strength of the voltage or current.

    A.C. voltages and currents are used because they are suitable for transformers,    (transformers can do nothing with D.C. and just short circuit).

    To measure A.C. we use a value that is 0.707 the peak value of one half cycle (whether positive or negative) this give us a number that shows the amount of work that A.C. will do when compared with an equivalent D.C. voltage or current.

    The important thing to remember is that if we are measuring across a transformer,  then the only voltage that will be there,  will be an A.C. voltage,   D.C. would be shorted out through the transformer windings.

    One thing that A.C. has,  that D.C. does not have,  is frequency,  the rate at which the positive changes to negative and back to positive is called the A.C. frequency,   for A.C. mains in this country this is 50 Hz,  that is the voltage changes around and back fifty times every second.

    When you decide to measure a voltage or measure a current,  you must first decide whether that voltage or current is A.C. or D.C.

    If the source of that voltage or current is a transformer,  then it is usually safe to decide that it is  A.C.,  and the A.C. ranges of the meter must be used for measurement.

    If however the source is a battery or D.C. power supply,  then it is very likely that the voltage or current to be measured is D.C.,  and the D.C. ranges should be used for the measurement.

    To decide whether it is the current or voltage that you are about to measure,   look at the circuit,  if the meter is in series with part of the circuit,  then the meter should be measuring the current flow through,  and should be set on a current range.

    If however the meter is to be set across part of the circuit,  then it should be set to a voltage range,  so as to measure the voltage across that part of the circuit.

    Remember always start with the meter set to the highest voltage/current likely to be encountered,   you can always set a lower range later.