A voltmeter is a measuring instrument that is used to measure the electrical potential difference between two points in an electric circuit. A rectangular voltmeter is a type of voltmeter that is designed with a rectangular-shaped display that shows the measured voltage. It is usually a digital voltmeter that uses a microcontroller or a specialized integrated circuit to measure and display the voltage.

The rectangular voltmeter may have additional features such as auto-ranging, data logging, and a backlight for use in low-light environments. It can be used in a variety of electrical experiments and measurements in physics and engineering laboratories.