A circuit is a distinct continuous range of motion of a mechanism. Changing circuits necessitates disassembling the mechanism. Thus, a designer must ensure a mechanism does not change circuits between two desired positions. A Watt chain is composed of two connected four-bar chains. A Watt chain may have one, two, three, or four circuits. The number of circuits and the extent of each circuit are shown to be determinable from the circuit attributes of the two component four-bars and by the limits of motion that these two four-bars place on each other. The circuit composition of all pin jointed Watt mechanisms is summarized in chart form. The chart enables detecting a change of circuit between any two trial positions of the same mechanism. The chart is suitable for both automated and manual analysis. Examples of circuit analysis of both Watt I and Watt II mechanisms are provided.

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