Trackballs are really upside-down, roller-bearing mice. Instead of the ball being inside the body, the ball is on the outside of a trackball and you move just the ball instead of the mouse body. Trackballs have several advantages over mice. Because you don't actually move the trackball itself, it takes up less space than a mouse does. And you don't have to lift it up to move it when you run out of room or reach. Because your hand remains stationary, you don't rub the sensitive areas of your wrist across the edge of your desk, which can lead to damage of the tissues in your forearm. Trackballs also have more than one button, and you can program the other buttons to perform various functions. For example, you can set a button to add a modifier key to the click so that you can bring up contextual menus with a click instead of having to hold down the Control key while you click. And trackballs can move the cursor either faster to cover more screen real estate or more slowly to give you more precise control than a mouse.
Choosing a trackball is similar to choosing a keyboard or mouse (except that Macs don't ship with a "default" trackball). Look for one that fits your hand and has the features you want?such as the number of buttons it has.
One of the best trackballs is Kensington's Orbit (see Figure 21.3). The Orbit has two buttons you can program, and you can even have those buttons perform different functions depending on the application you are using. This is managed through the excellent Kensington MouseWorks software. For example, I have my Orbit set to click the left button as a standard single-click and the right as a double-click, and both as a Control-click for contextual menus.
You can learn more about Kensington input devices at www.kensington.com.
Installing trackballs is also similar to installing keyboards and mice. You attach the device through an available USB port and then install and configure its software.
If you experience fatigue or pain when using any input device, make sure that you experiment to see whether you can find a more comfortable position for the device. If you can't, consider replacing the device with another type that is more suited to you. Discomfort, even of a mild nature, can indicate that some damage is being done to your body. If this happens over a long period of time, you can end up with serious health problems.
It's a good idea to have several different devices and positions among which you can rotate so that you can avoid repeating exactly the same actions over an extended period of time. For example, you might want to have both a mouse and a trackball and switch between those devices every so often.