Which of the following is not measured by any kind of voltmeter?

Concept:

Voltmeter:

• It is a device used to measure potential difference and is always put in parallel with the ‘circuit element’ across which potential difference is to be measured

Conversion of galvanometer into voltmeter:

•  A galvanometer may be converted into a voltmeter by connecting a large resistance R in series with the galvanometer. Therefore option 4 is correct.
• The equivalent resistance of the combination = G + R
  Where G = resistance of galvanometer.

Copper Voltmeter

• A copper voltameter is a device for measuring the electrochemical equivalent of copper
• It is a measuring instrument used for measuring the quantity of electricity (electric charge) by the electrolytic action.

Electrosatic voltmeter

• Electrostatic is a device to measure large electrical potentials.
• It is also known as an electrostatic charge meter.
• The mass of the substance deposited to one of the electrodes when a current of 1 ampere is passed for 1 second or a quantity of electricity of one coulomb is passed is called electrochemical equivalence. 

Explanation:

• Current is not measured by a voltmeter. 
• It is measured by an ammeter in which a small resistance R is kept in parallel to the galvanometer. 

So, the correct option is Current

Additional Information

• A galvanometer can be converted into an ammeter by connecting a shunt resistance in parallel to it.
• The shunt resistance should have very low resistance. So, the ammeter (the parallel combination of galvanometer and shunt resistance) will have low resistance.
• To convert a galvanometer into an ammeter of current rating ‘I’, a small resistance ‘S’ (shunt resistance) is connected in parallel across the galvanometer.