Voltage

Here’s why:

The Circuit in 25.18b is effectively an open circuit. Modern volt meters can have input impedances in the billions of ohms (even the cheap ones are in the tens of millions of ohms). Because this circuit is effectively open, there is no current flow, so there is no voltage drop on the resister. Without a voltage drop, the source voltage is “read” by the meter.

There are many voltage sources that require a “load” to be accurately measured. Without a load, the source voltage may be several times higher than the specified rating.

_7UP_

ok, but why in the circuit 25.18b  Vaa' = 0  and doesn't equal to Vab ? 

An ideal amp meter is a conductor and has zero impedance, so there is no potential difference hence no voltage drop, whether current is passing through or not.

V_ab will always have voltage (if the supply is working).

_7UP_

315, you are being a bit stingy with your thank yous and your points here. :/

People want to see tokens of appreciation!

So if we assumed that there is no ameter between a and a' so Vaa' will equal Vab ? 

Melody, I didn't finish this topic yet because in the last two days I done other exams so I left the physics for couple days =D  

Yes. In circuit A, If the amp meter is removed and a volt meter is connected in series (the same as in circuit B) then Vaa' will equal Vab which is the “unloaded” source voltage.

_7UP_

Thank you 7up 

Thank you sevenup and 315 :)