QUESTION: What causes the screen on my iPad to rotate when I turn it? How does it do that? (Asked by a column reader with an iPad.)

REPLY: One of the most interesting features of a tablet (iPad) is the ability to “flip” or rotate screen images from up-and-down to horizontal (portrait to landscape mode), and vice versa. It is rather complicated, but I’ll give it a ruddy go.

What is the internal mechanism responsible for this? The answer is an accelerometer. Acceleration is the time rate of change of velocity. It is a vector, having magnitude and direction. A speedometer measures speed (a scalar), whereas an accelerometer measures acceleration (a vector with direction).

There are various types of accelerometers. To learn the basics of operation, let’s look at a simple mechanical accelerometer. Consider a spring with a suspended mass (a steel ball) attached to a housing or container. When the housing is moved upward, the heavy ball lags behind, stretching the spring. From the degree of stretch, the acceleration because gravity can be calculated. The direction of motion in this case is known, but indicated by the accelerometer. Two accelerometers perpendicular to each other could be used to define two dimensional directions or a plane on rotation. You might have noticed there is no rotation when an iPad is held horizontally and rotated. (Also, you might have noticed that opening smart phone icon screens do not generally rotate, and only certain apps do – those having accelerometer applications. For example, the calendar and calculator apps on my iPhone rotate. Check your smart phone out.)

Of course, there aren’t mechanical accelerometers in a tablet. But, there are tiny silicon accelerometers on chips. Components on the chip provide motion-generated electrical signals from which the acceleration and orientation may be measured. Circuitry then causes the screen to rotate on a perpendicular axis. Once orientated, the process and rotation may be reversed.

Accelerometers are put to use in a variety of applications that measure rotation, collision and vibration. For example, accelerometers are used in airbag inflation in auto collisions. Electronic signals cause the bags to inflate when there is an acceleration detected -- in this case, a deceleration (negative acceleration) caused by the collision.

C.P.S. (Curious Postscript): “I’m so fast that last night I turned off the light switch in my hotel room and was in bed before the room was dark.” -- Muhammad Ali

Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or e-mail jerry@curiosity-corner.net. Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to curiosity-corner.net.