Scientists hypothesized that finer grain iron oxide on video and audio tapes would result in better recording and playback. They produced ten tapes with a range of partial sizes, recorded them, and tested the quality of the playback. Electronic instruments and human visual and audio reaction were used to determine whether there was any significant difference between the tapes with different particle sizes of iron oxide

What is the constant variable? Because I think there's none but I'm not sure if I'm right

In the given experiment, a constant variable refers to a factor that remains the same throughout all the samples being tested. It is used to ensure that any observed differences in the results can be attributed solely to the manipulated variable (in this case, particle size of iron oxide).

Based on the information provided, it appears that the researchers manipulate the particle size of iron oxide on video and audio tapes and then test their quality of playback. In this experimental design, the researchers have not explicitly mentioned any constant variable. However, it is important to note that there might be other factors that were kept consistent, even if not explicitly mentioned in the question.

To further confirm whether there is a constant variable, we would need additional information about the experimental setup and procedures outlined by the scientists.