What is a transformer? What is the function of a step-up transformer?

A transformer is a device that converts high voltage AC to low voltage AC of the same frequency and vice versa The function of the step-up transformer is that it converts the low voltage of current to high voltage.

Which symbols are used in the story and what do they indicate?

the symbols used in the story are:

1. house and shops- it symbolizes Aksionov's family and material possessions
2. nightmare-it symbolizes a bad omen.
3. blood-stained knife- it indicates Aksionov's lack of control over his fate.
4. the prison- Aksionov's pain and subsequent mystical transformation are symbolized by the prison.
5. the book(the lives of the saints)- gives an impression of religiosity.

Why can't a transformer be used to change the value of DC voltage? 

A transformer relies on the principle of electromagnetic induction, which works with changing magnetic fields. Since direct current (DC) has a constant magnitude and does not change direction, it does not create the varying magnetic field necessary for induction in a transformer. Therefore, transformers are not suitable for altering the value of DC voltage.