Is the partial credit model a Rasch model?

A balance scale metaphor is offered as a tool for explaining the principles of measurement and for visualizing the internal structure of dichotomous and polytomous Rasch models. The balance scale metaphor is used to guide the derivation of a general polytomous Rasch model and to illustrate the additional assumptions subsequently required to derive the Andrich (1978) rating scale model (RSM) and the Masters (1982) partial credit model (PCM). The metaphor is used to present the argument that the RSM conforms to the rules of measurement, but the PCM has interactions implicit in its structure that violate specific objectivity and sufficiency of raw scores, which challenge its status as a Rasch model. Using the metaphor and a literal interpretation of the narrative description of the PCM by Masters (1982), a new version of the PCM is derived that does conform to the rules of measurement.