found someone who could explain the taps to her. A simple sink, asimple-looking faucet. But it looks as if it should be turned, notpushed. If you want the faucet to be pushed, make it look as if itshould be pushed. (This, of course, is similar to the problem I hademptying the water from the sink in my hotel, described in Chapter 1.)Why is such a simple, standard item as a water faucet so difficultto get right? The person using a faucet cares about two things:water temperature and rate of flow. But water enters the faucetthrough two pipes, hot and cold. There is a conflict between thehuman need for temperature and flow and the physical structureof hot and cold.There are several ways to deal with this:Control both hot and cold water: Two controls, one for hot water,�the other cold.Control only temperature: One control, where rate of flow is fixed.�Rotating the control from its fixed position turns on the water atfour: Knowing What to Do: Constraints, Discoverability, and Feedback 151some predetermined rate of flow, with the temperature controlled bythe knob position.Control only amount: One control, where temperature is fixed, with�rate of flow controlled by the knob position.On-off. One control turns the water on and off. This is how gesturecontrolled�
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faucets work: moving the hand under or away fromthe spout turns the water on or off, at a fixed temperature and rateof flow.Control temperature and rate of flow. Use two separate controls, one�for water temperature, the other for flow rate. (I have never encounteredthis solution.)One control for temperature and rate: Have one integrated control,�where movement in one direction controls the temperature andmovement in a different direction controls the amount.Where there are two controls, one for hot water and one for cold,there are four mapping problems;Which knob controls the hot, which the cold?�How do you change the temperature without affecting the rate of�flow?How do you change the flow without affecting the temperature?�Which direction increases water flow?�The mapping problems are solved through cultural conventions,or constraints. It is a worldwide convention that the left faucetshould be hot; the right, cold. It is also a universal convention thatscrew threads are made to tighten with clockwise turning, loosenwith counterclockwise. You turn off a faucet by tightening a screwthread (tightening a washer against its seat), thereby shutting offthe flow of water. So clockwise turning shuts off the water, counterclockwiseturns it on.Unfortunately, the constraints do not always hold. Most ofthe English people I asked were not aware that left/hot, right/cold was a convention; it is violated too often to be considered aconvention in England. But the convention isnt universal in the�152 The Design of Everyday ThingsUnited States, either. I once experienced shower controls that wereplaced vertically: Which one controlled the hot water, the top faucetor the bottom?
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