Why are currents "measured" on the p pin?

[dodoplus]

Take a look at this simple example - it's the same as in the docs, but with a Current source instead of a Voltage source:

using ModelingToolkit, OrdinaryDiffEq, Plots
using ModelingToolkitStandardLibrary.Electrical
using ModelingToolkitStandardLibrary.Blocks: Constant

R = 1.0
C = 1.0
I = 1.0
@variables t
@named resistor = Resistor(R=R)
@named capacitor = Capacitor(C=C)
@named source = Current()
@named constant = Constant(k=I)
@named ground = Ground()

rc_eqs = [
        connect(constant.output, source.I)
        connect(source.p, resistor.p)
        connect(resistor.n, capacitor.p)
        connect(capacitor.n, source.n, ground.g)
        ]

@named rc_model = ODESystem(rc_eqs, t, systems=[resistor, capacitor, constant, source, ground])
sys = structural_simplify(rc_model)
prob = ODAEProblem(sys, Pair[], (0, 10.0))
sol = solve(prob, Tsit5());

if we inquire the current at the source we get:

sol[source.i]
7-element Vector{Float64}:
1.0
 1.0
 1.0
 1.0
 1.0
 1.0
 1.0

which makes sense.

However, the current on the resistor is:

sol[resistor.i]
7-element Vector{Float64}:
 -1.0
 -1.0
 -1.0
 -1.0
 -1.0
 -1.0
 -1.0

which looks weird.

The current on the resistor should flow in the direction p -> n, and thus be positive.
However, resistor.i is an alias for resistor.p.i which measures the current in the reverse direction.

We could alias i as n.i in OnePort which will then measure the current out of the component.

[ValentinKaisermayer]

Both are subtypes of a OnePort and follow the Modelica standard library. The sign convention is that the flow variable (in this case the current) is positive if the flow flows in the component. And the convention for the pin is "Positive pin (potential p.v > n.v for positive voltage drop v)".