Theory of Ammeters: Does it Affect Current Measurement?

[cupid.callin]

<Pic> This is the theory of ammeter give in a book.
please read it.

Now, my question is that coil will only respond to the current through it i.e. i₁.
But this is a very small part of total current (R_c >> r).
Does this mean that all the moving coil ammeters show wrong current?

Please help me.

 

[Zryn]

The deflection of the needle is proportional to the current i1 through it, which is proportional to the total current i. So if i1 is 1A and moves the needle 1mm, and i1 is 1/1000th of i, the scale on the piece of paper behind the needle will show 1000A for each 1mm increment of the needle.

There will always on any measurement device be an estimate of error depending on how precise the instrument is. It could be 0.001% or 1% or 10%. 'Wrong current' is only a matter of precision in most cases. If your ammeter can tell you current to 3 decimal places (mA) and you need it accurate to 12 decimal places (pA) then you just need to get a better ammeter, your ammeter isn't necessarily wrong, merely operating within its limits of its estimate of error.

 

[cupid.callin]

Ok! well that seems to be a satisfying answer!

Thank you for your help Zryn!
really appreciate it~~!