Why does a voltmeter have very high impedance and an ammeter very low impedance

A voltmeter is designed to measure the voltage difference between two points in an electrical circuit. To do this without affecting the circuit itself, it needs to have a very high impedance. Impedance is a measure of how much a device resists or impedes the flow of electric current. A high impedance means that very little current will flow through the voltmeter. This is important because if significant current were to flow through the voltmeter, it would alter the voltage difference it's trying to measure.

On the other hand, an ammeter is designed to measure the current flowing through an electrical circuit. It does this by being placed in series with the component of the circuit it's measuring. In order not to affect the current it's measuring, it needs to have a very low impedance. A low impedance means that current will flow through the ammeter with very little resistance. If the ammeter had a high impedance, it would impede the flow of current and alter the measurement.