A voltmeter or a multi-meter.
I presume you mean what do you measure electric current with. You would use an ammeter.
"ammeter"
A current transformer is used for measurement of alternating electric currents. Current transformers, together with voltage transformers are known as instrument transformers.
That would depend on which electrical quantity you need to test and measure. It might be voltage, current, resistance, charge, inductance, capacitance, frequency, power, energy, reactance, impedance, admittance, susceptance, conductance, permeability, permittivity, resistivity, conductivity, etc., etc. There's no single test instrument that can measure all of them. The design of the instrument, and the components, materials, and tools needed to build it, are different for each different instrument.
The ceilometer instrument.
A tape measure.
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No, an example of an electric current would be using a battery to light a lightbulb.
It would depend on what type of frequency you are measuring. One instrument in electric and electronic engineering would be an oscilloscope.
it would be a ruler
Balanced .
If the circuit is carrying current then that means that the load (resistance) is in the circuit. if an ohm meter is connected in the live circuit then there would be some voltage drop at the ohm meter but as the meter has very less resistance, this would damage the instrument.