I do start to question the transformer setup. Do you have a picture of it? If no, can you describe it further, which kind is the primary connections? Is the primary connection fixed or through power cord, which you can freely attach. Is the transformer in the box or open setup on the laboratory environment, do you see both the primary and secondary connections?
Which level of education system you are currently studying, I do ask so I can judge how much background information you might have and how detailed information you really need.
Draw or take a picture, it would clarify the situation. In general this is really super simple if the secondary of the transformer is floating / galvanically isolated. Easy if the transformer is not galvanically isolated, but the mains connection is fixed to the wall / laboratory table. A bit more interesting if the transformer is connected with typical three pin schuko connector and the secondary is grounded.