Frequently what is needed in practice is a measurement of the phase dillerence between two signals of the same frequency: that is, it is necessary to measure the relative phase between two signals rather than the absolute phase of either one(see Figure 41.2).Often, in the measurement of the relative phase between two signals, both signals are derived irom the same source. These signals might, for example, be the current and voltage of a power system: the relative phase, o, between the current and voltage would then be useful for monitoring power usage, since the latter is proportional to the cosine of o Several techniques are available for the measurement of relative phase. One crude method invoives forming"Lissajous figures" on an oscilloscope. In this method, the first of the two signals of interest is fed into the vertical input of a CRO and the other is ied into the horizontal input. The result on the oscilloscope screen is an ellipse, the intercept and,maximum height of which can be used to determine the relative phase. Other methods for determining relative phase include the crossed-coil meter(based on electromagnetic induction principles), the zero-crossing phase meter(based on switching circuitry determining the fractional portion of the period completed), the three-voltmeter method(based on the use of three signals and trigonometric relationships), and digital methods(based on analog-to-digital conversion and digital processing)