Selected classes of positive electrical circuits and their reachability

Conditions for the positivity of linear electrical circuits composed of resistors, coils, condensators and voltage (current) sources are established. It is shown that: 1) the electrical circuit composed of resistors, coils and voltage source is positive for almost all values of their resistances, inductances and source voltages if and only if the number of coils is less or equal to the number of its linearly independent meshes, 2) the electrical circuit is not positive for any values of its resistances, capacitances and source voltages if each its branch contains resistor, capacitor and voltage source, 3) the positive n-meshes electrical circuit with only one inductance in each linearly independent mesh is reachable if all resistances of branches belonging to two linearly independent meshes are zero 4) the electrical circuits of the structure shown on Fig.5.2 is reachable if and only if the condition (5.11) is satisfied.