# What Is an Experimental Constant?

## Explanation and Examples of Constants

A constant is a quantity that does not change. Although you can measure a constant, you either cannot alter it during an experiment or else you choose not to change it. Contrast this with an experimental variable, which is the part of an experiment that is affected by the experiment. There are two main types of constants you may encounter in experiments: true constants and control constants. Here is an explanation of these constants, with examples.

### Physical Constants

Physical constants are quantities which you cannot change. They may be calculated or defined.

Examples: Avogadro's number, pi, speed of light, Planck's constant

### Control Constants

Control constants or control variables are quantities a researcher holds steady during an experiment. Even though the value or state of a control constant may not change, it is important to record the constant so the experiment may be reproduced.

Examples: temperature, day/night, duration of a test, pH