DC Voltage on Oscilloscope Without Time-Base

Steps:

• DC power supply outputs constant voltage, causing steady vertical deflection of the electron beam.
• Time-base switched off stops horizontal sweep, so beam does not move left to right.
• Beam remains at fixed vertical position while potentially tracing horizontally due to persistence or slight drift.
• Screen displays a steady horizontal line at the voltage level.

Why A is correct:

• Constant DC voltage with no time-base sweep deflects beam uniformly vertically, per oscilloscope principle of Y-deflection for voltage and X for time.

Why the others are wrong:

• B: Vertical line requires time-base on with constant voltage, showing no horizontal variation.
• C: Spot appears only if beam is fully stationary without trace; DC typically shows line due to beam intensity.
• D: Nothing implies no signal or deflection, but DC voltage produces visible trace.

Final answer: A