Given a scientific passage and an answer, generate a question for the given answer.
One example: Passage: A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:. Answer: nucleotides.
Solution is here: A frameshift mutation is a deletion or insertion of one or more of what that changes the reading frame of the base sequence?
Explanation: The science passage defines frame mutation and the main compoment here is nucleotides. Hence the generated question is apt.

Now, solve this: Passage: 21.4 DC Voltmeters and Ammeters Voltmeters measure voltage, and ammeters measure current. A voltmeter is placed in parallel with the voltage source to receive full voltage and must have a large resistance to limit its effect on the circuit. An ammeter is placed in series to get the full current flowing through a branch and must have a small resistance to limit its effect on the circuit. Both can be based on the combination of a resistor and a galvanometer, a device that gives an analog reading of current. Standard voltmeters and ammeters alter the circuit being measured and are thus limited in accuracy. Answer: voltage
Solution:
What do voltmeters measure?