No.4: A voltmeter can measure negative ions if it can detect extremely low voltage.

There are small and large ions in the air which are classified depending on the mass. They produce different effects on the human body. Generally, it is said that small ions have much more beneficial effects.

Electronically, a voltmeter cannot tell if it is a small or large ion because both ions posses the same amount of electricity that an electron does. This is why you need a specially designed instrument particularly to measure small ions which produce more beneficial effects on the human body.