Can the apparatus used be our controlled variable?

You cannot use the ruler as a variable, it is just one of your materials.

Examples of controlled variables in your experiment could be the soil the plants are growing in, the temperature of the room, and the number of hours of light vs. dark, as long as these variables were kept the same for all your groups.

The one variable you change (let's say the number of fertilizer pellets added to each plant) is your independent variable.

The variable that is affected by this change (let's say the height of the plants) is your dependent variable. The dependent variable changes in response to the changes you make in the independent variable. You measure these changes with your ruler, but the ruler itself would not be a variable.

A ruler uses length. Length is a constant i.e. it doesn't matter what ruler you use to measure 1cm, it will still be 1cm. So the choice of ruler is kind of meaningless. If you are talking about your CONTROL variable, then this is important.

For example if you are comparing the effect of a fertiliser on the height of plants then your control variable would be - plant height without fertiliser. Your experimental/treatment group would be the height of plants grown in the presence of fertiliser. Dependent variable= Plant height. Independent variable=fertiliser or not. In this case, if I was talking about which variables I controlled, I would say something like: Plants were randomly assigned into each experimental group, with 10 plants in the group treated with fertiliser and 10 in the experimental group grown in the absence of fertiliser. (this controls for human bias). Plants were grown in the same environmental conditions (eg in a greenhouse, same amount of water, light etc) and measured daily at defined points throughout the day. etc etc etc these are variables which you try to control but the ruler is not one.

If you were using a machine to measure the height, say, then you would just state what kind of machine was used. But if you used one machine, then it broke and you had to measure the rest of the plants by a different machine, then you would have to state this - because if the machine added 1cm to every result you obtain then it breaks and you get a machine that works (doesn't add 1cm to results) then you would be liable to think that the second set of plants are shorter even though it was a result of using different measuring devices.