tradicts the conceptual model presented by the refrigerator’s system image—the diagram on the refrigerator itself.

What must be emphasized in any discussion of conceptual models is the knowledge that users bring with them from previous experiences with mechanical devices and information systems. When humans engage in learning, they attempt to assimilate the new experience into previously formed mental representations of reality. If the new experience matches those representations, learning occurs more easily. If there is a mismatch, real learning can occur only when the person alters his or her mental representations of reality in some way to accommodate the new experience. One can see how damaging a false system image can be to this process: Rather than strengthening a person’s understanding of the world, the new learning experience corrupts it. We must always consider the previously formed mental representations of the world that people bring with them to new experiences with mechanical devices and information systems, for these inform their perception of the system’s image and therefore the conceptual model they perceive as part of their experience.

Telegraph operators of the 19th century would have understood Bell’s box telephone immediately. Because of their prior experiences, they had already built mental models that could assimilate the idea of electrical current traveling along a wire; the need for a closed circuit between two devices on the wire and the need to interrupt that circuit under certain cir-

cumstances; and the ability to convert electrical pulses into something else at the ends of the wire, such as the swings of a needle on a galvanometer, the series of dots and dashes in Morse code, or sound waves in the telephone. Telegraph operators also would have understood the knob and crank of the Bell Box as a way of generating current along the wire and ringing a bell to indicate at the other site that a response is requested. The plugs to open and close the circuit would likewise fit well with their mental picture of such instruments. For those who had prior experiences with it, the telegraph served as a valuable antecedent in the new experience of using the early box telephones.

But to people of the 1870s who did not have prior intimate experience with the telegraph, the new telephone was magical and frightening. Merritt Ierley notes, “the reaction was confusion or disbelief. Many people were apprehensive confronting a telephone for the first time. The disembodied sound of a human voice coming out of a box was too eerie, too supernatural, for many to accept [ 1].” Only when the telephone became understood as a “speaking telegraph” did the masses became more comfortable with it. But this is merely another way of saying that people learned to assimilate the telephone into their established mental picture of how such devices work. And this could happen only when the everyday operation of the telephone became more like everyday uses of the telegraph. Ordinary people did not operate a telegraph machine. They

handed their messages to the clerk, and he or she sent them along the wires. Likewise, the maturation of the telephone— the improvements in design that made it useful as an everyday appliance—took much of the operation of the apparatus out of the hands of the user, making it much easier for ordinary people to learn how to make a telephone call. Even the character of the first telephone conversations was determined by users’ prior experience with the telegraph. Explaining their brevity, Ierley writes: “the telegraph was understood to be a medium for short, to-the-point, business-like messages. So too, it seemed, the telephone [ 1].”

Our goals have not changed in the past hundred years and more. We want to write, to communicate, to buy and sell goods and services, to move from one place to another, to understand a problem, to make good decisions. But the technologies that help us achieve these goals do change, sometimes quite rapidly. They are “contingent” technologies in at least two important and connected ways. First, our ability to learn and use new technologies is contingent upon our experience with prior technologies. On a computer, for instance, each time we learn a new interaction idiom such as drag-and-drop, or double clicking, or scrolling, we adopt new ways of understanding how software applications and hardware devices work. We compare these new experiences to past ones; we recognize solutions to problems that previously puzzled us; we assimilate the new experience and store our new understandings so that they will serve as helpful anteced-