Tony, me thinks you need to go to FIRST principles regards meters on HOW they operate. To measure voltage, a series resistor is fitted series with meter movement and this rudimentry meter applied at any two points where voltage needs to be measured. For DC, the circuit through which current flows is cut and rudimentry meter placed between the cut wires so that all the current flows through the rudimentry meter- only in this case the 'heavy' current flow is bypassed through the shunt. I know, you already know all that. But what I cannot understand is, how can you post a picture and state this is how you would measure alternate current. Clealry the following says otherwise
Your drawing is to be found here!!
http://www.allaboutcircuits.com/vol_2/chpt_12/1.html
Clearly the drawing you have cut and pasted is for voltage indication
The text reads
The reason why a bridge rectifier is used is to present rectified voltage to the meter ELSE the meter movemet will OSCILLATE.
You might want to look at following for ammeter design for DC current measurement
http://www.allaboutcircuits.com/worksheets/meters2.html
Or
http://www.richardsradios.co.uk/Images/mk2diagram.jpg
which is AVO 8 showing both DC and AC current measurement.
Its so simple but here is a circuit:-
Your drawing is to be found here!!
http://www.allaboutcircuits.com/vol_2/chpt_12/1.html
Clearly the drawing you have cut and pasted is for voltage indication
The text reads
In order to use a DC-style meter movement such as the D'Arsonval design, the alternating current must be rectified into DC. This is most easily accomplished through the use of devices called diodes. We saw diodes used in an example circuit demonstrating the creation of harmonic frequencies from a distorted (or rectified) sine wave.
The reason why a bridge rectifier is used is to present rectified voltage to the meter ELSE the meter movemet will OSCILLATE.
You might want to look at following for ammeter design for DC current measurement
http://www.allaboutcircuits.com/worksheets/meters2.html
Or
http://www.richardsradios.co.uk/Images/mk2diagram.jpg
which is AVO 8 showing both DC and AC current measurement.