A current measuring instrument is called an 'ammeter'. A 'galvanometer' is an earlier form of a current measuring instrument. However any gadget that 'reacts' or responds to a current is not necessarily a good ammeter. All measuring instruments in general have to meet one criterion - they cannot disturb or alter the physical quantity being measured. This for an ammeter means that it cannot change the current in the circuit.
Consider for example the circuit below. Here
I = E / R
Modify the circuit to include an ammeter (Note ammeters are always used in series - they have to pass all the current that you are trying to measure)
Then the new current I' = E/(R + r). This current is approx equal to the original current only if r << R. The fractional error in measuring the current is dI/I = r/R.
Voltmeters - Voltmeters are always used in PARALLEL as shown in the figure below. Requirement on a voltmeter is that it should not disturb the current through R. Here:
I = E/R and I'=E/Req
Fractional error is dV/E =
For Rv >> R the fractional error is small. Typical voltmeters have Rv ~ 10 -100 Megaohms.