Electrometer

An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made instruments to high-precision electronic devices. Modern electrometers based on vacuum tube or solid state technology can be used to measure extremely small currents down to 1 femtoampere.

Experiments with an electroscope

• Touch the terminal with a charged conductor. Electrons flow into or out of the electroscope until its potential equals that of the conductor. The net surplus or deficit of electrons on the leaves causes them to repel each other. The charge that transfers, and therefore the deflection, are functions of the original potential difference between the conductor and the uncharged electroscope.
• Place an isolated, charged conductor in contact with the terminal. The proportion of the conductor's charge that ends up on the terminal depends on their relative capacitances. This charge remains on the electroscope when you take the conductor away. The electroscope indicates this charge.
• Bring a charged insulator near to the terminal but don't allow any charge to transfer between them. In this case, the deflection indicates the net charge on the electrons that have been temporarily pushed towards or away from the leaves. This charge is related to the charge density on the part of the insulator near to the terminal, and the relative shapes and positions of the two. What the electroscope is measuring in this case is quite vague, because there is no sharp boundary between the electron-rich and electron-depleted regions of the electroscope.
• Place a charged insulator on or near the terminal, ground the terminal and then withdraw the insulator. This leaves the electroscope with a net charge. The electrons redistribute themselves over the terminal and the leaves, causing the leaves to repel each other. The deflection is monotonically related to the net charge left behind on the electroscope, in a similar way to the previous case.