When using the linear time-base method in eddy current testing, what is applied to the vertical deflection plates when conditions are balanced?

In the linear time-base method of eddy current testing, the instrument's vertical deflection plates are specifically set to respond to the balanced conditions of the system. When these conditions are achieved, there is no need for any voltage input to drive the vertical deflection plates; thus, a zero net voltage is applied.

This zero voltage condition signifies that the detected signal is completely balanced against the reference signal, meaning that any variations or signals detected in the material under investigation are accurately represented without distortion. The intent in this scenario is to ensure that the display of the eddy current signal on the oscilloscope or testing instrument shows only the relevant data reflecting the material's properties, without extraneous influences that would occur if a different voltage were applied.

The other options, such as sinusoidal waves in or out of phase with the timing voltage or a sawtooth waveform, would indicate scenarios where a signal is present and would lead to an oscillation or distortion in the displayed results. This would not demonstrate the balanced condition necessary for accurate readings in this testing methodology.