Hysteresis

When shaving with a multiple-blade razor, the first blade cuts into the whisker while simultaneously pulling it out slightly. Before it can retract, subsequent blades cut the hair even closer, resulting in whiskers which end up below the surface of the skin.

According to Cecil Adams of The Straight Dope, Gillette claims to have slow-motion microphotography demonstrating hysteresis.

Sources:

The Gillette Company Web Site:

http://www.gillette.com

The Straight Dope:

http://www.straightdope.com

The Greek root hyster means uterus. It was believed that the Moon could tug on the womb as it does the oceans, resulting in erratic behavior in women. It is for this reason that we have the term Hysteria. Not surprisingly, the preferred cure for this condition has traditionally been the hysterectomy, or removal of the uterus.

The notion of a shifting uterus has been a convenient analogy to use to describe unrelated phenomena, and it's for this reason that we have the word hysteresis. In electronic parlance, hysteresis can be a built-in "dead zone" between two logical states.

A good example of this is present in the thermostats that control air conditioners, heaters, and refrigerators. On a hypothetical Summer day, we set the desired temperature to 74 degrees Fahrenheit. In theory, if the measured room temperature is above 74, the thermostat turns the air conditioner on. If the temperature is below 74, the thermostat turns the machine off. If there is no hysteresis in the system, the thermostat will very rapidly toggle on and off around the 74 degree mark to maintain the temperature. That would be undesirable because of the mechanical stress to the unit and wasted energy.

For that reason, all thermostats have some hysteresis built in. If the user sets the desired temperature to 74, the thermostat will keep a margin of one or two degrees on either side of that value to use for the trigger points. So, if the temperature falls below 72 degrees, for example, the AC is turned off, and if it rises above 76, the AC is turned back on. Between these two temperatures, no action is taken.

Hys`te*re"sis (?), n. [NL., fr. Gr. to be behind, to lag.] Physics

A lagging or retardation of the effect, when the forces acting upon a body are changed, as if from velocity or internal friction; a temporary resistance to change from a condition previously invuced, observed in magnetism, thermoelectricity, etc., on reversal of polarity.

© Webster 1913.