Tri-state area

There are a number of areas in the 48 contiguous United States known as tri-state areas where three states either meet at one point (a tripoint) or are in close proximity to each other. The two best known of the latter type are for the New York and Chicago metropolitan areas. Two well-known examples of the former are Philadelphia's Delaware Valley and the Pittsburgh Tri-State.

The Tri-State Region around New York City is where New York, New Jersey, and Connecticut nearly meet. There is no tripoint, however, as Connecticut does not touch New Jersey. Chicago's tri-state region includes Illinois, Indiana, and Wisconsin and is commonly referred to as Chicagoland. This tri-state region also does not have a tripoint since Indiana does not touch Wisconsin. Pittsburgh's tri-state region includes Pennsylvania, Ohio, and West Virginia, which meet near the Beginning Point of the U.S. Public Land Survey northwest of Pittsburgh. Philadelphia's tri-state area consists of Southeastern Pennsylvania, Southern New Jersey, and Delaware, with a tripoint meeting in the Delaware River near the east end of the Twelve-Mile Arc.

Other large metropolitan areas with a tri-state area include Cincinnati and Memphis. Smaller ones include those of Dubuque, Iowa, which spills over into Illinois and Wisconsin; and of Evansville, Indiana, which includes parts of Illinois and Kentucky.

The area that includes Washington, D.C. and the nearby parts of Maryland and Virginia is sometimes loosely referred to as a "tri-state area," although Washington is not a state.

The "Joplin District", a lead and zinc mining region of Oklahoma, Kansas and Missouri, produced mineral specimens known as "Tri-State" minerals.

Positive feedback

Positive feedback, sometimes referred to as "cumulative causation", refers to situations where some effect causes more of itself. A system undergoing positive feedback is unstable, that is, it will tend to spiral out of control as the effect amplifies itself.

Technically, a system exhibiting positive feedback responds to perturbation acts to increase the magnitude of a perturbation. That is, "A produces more of B which in turn produces more of A". In contrast, a system that responds to the perturbation in the opposite direction is said to exhibit negative feedback. These concepts were first recognized as broadly applicable by Norbert Wiener in his 1948 work on cybernetics.

The effect of a positive feedback loop is usually not "positive" in the sense of being desirable. Positive refers to the direction of change rather than the desirability of the outcome. A negative feedback loop tends to reduce or inhibit or stabilise a process, while a positive feedback loop tends to expand or promote it and will often ultimately destabilise it.