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A demonstrator model (also called a demonstrator) is an example of something (most often a physical object) used in some kind of presentation or exhibit.
In some cases, a standard or production version of the product will be used; the salesperson simply carries the product around, and may even sell the item they are presenting and replace it with another.
In other cases, special variants of the product will be used. These have features designed to aid in sales, such as having sections of the product manufactured so as to make the internals of the product more visible, often through the use of transparent plastics. One example of this type of demonstrator is the demonstrator pen, a partially (or wholly) transparent sales demonstrator for fountain pens. Numerous camera demonstration models have been made, as well as other small manufactured mechanical objects. These can easily become collectibles, as they are both rare (compared to the object that they are demonstrating) and can be interesting to look at.
In cases where a product may be non-portable, custom-built, not yet completed, or too expensive to allow for a unit to be used only for demonstration purposes, a scale model or non-functional full-size mockup may be used. IBM at one time used a sort of diorama of a room-sized mainframe computer. This demonstrator was packaged in its own briefcase and could be carried on sales calls, then set up to show a miniature version of the large multi-million-dollar device.
In the case of large items such as aircraft, automobiles, or railroad rolling stock, the manufacturer may designate one or a small number of the units as dedicated demonstrators, though they may be later sold to customers. Sometimes these units are prototypes or examples from the early stages of series production, and may not be identical to later units. Frequently demonstration models of this sort will be the fanciest versions of the products, with extra or optional features included in order to show off those features and to entice buyers.
Especially in science classrooms, but also in many other fields and locations, models are used either as visual aids or to demonstrate a principle or process. For example, small Jacob's Ladder spark gap devices are used in lessons about electricity, and models of basic devices such as blocks and tackle are used to show certain physics principles.