3. A student is investigating factors that affect the strength of an electromagnet. The student proposes the following design.
Factors: Voltage, number of turns of wire.
Process: Set voltage to 2.0 V, wrap 50 turns of wire around an iron coil. Measure how many paper clips the magnet can pick up. Set voltage to 4.0 V and wrap 30 more turns of wire. Again measure the number of paper clips. Repeat several times.
Which of the following is the most significant problem with this experimental design?