A simpler way of defining voltage and current observables is using "Probes". [[B2.Spice A/D]] provides two types of probes: Voltage Probes and Current Probes. Both can be accessed from the [[Schematic toolbar|Schematic Toolbar]] at the top of the screen or by selecting "Probe Mode" in the [[Schematic Editor]]'s Edit Menu. The keyboard shortcut "Ctrl+P" can be used for this purpose, too. While in the Probe Mode, you can place as many probes as you want. Voltage probes must be placed at the circuit nodes or on the wires, while current probes must be placed on a device pin. Voltage signal are always measured with respect to the circuit's ground. The direction of a current signal is determined by its underlying device pin, which shows the current's entrance into the device. The opposite figure shows a typical operational amplifier (Op-Amp) circuit with two voltage probes placed at its input and output nodes. The figure below it shows a graph of the plotted probe voltages.
[[File:b2MAN_Fig110.png|thumb|left|600px|A Cartesian graph showing the input and output voltages of the Op-Amp circuit.]]
[[File:b2MAN_Fig110.png|thumb|left|600px|A Cartesian graph showing the input and output voltages of the Op-Amp circuit.]]
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