To me, it's important to know how far I've travelled and at what speed. In the last few years cycling has come a long way from the old Sachs belt-driven odometers. Today, the cyclist has a variety of cycling computers available for surprisingly little outlay. Many offer features such as speed, max speed, average speed, trip distance, overall distance and clock.
The model I use, a Sigma BC800, offers all these functions in one small unit. Drawing power from a 1.5V lithium watch battery, it's waterproof, accurate and easy to use and set up. For security, it merely slides off its' mounting bracket and pops in your pocket.
Sensing of speed is by means of a hall-effect sensor - a semiconductor device which is designed to generate a voltage when exposed to a moving magnetic field. A magnet is mounted on the wheel spokes, and is sensed by a sensor affixed to the inside of the forks. The two devices do not touch, and therefore are completely silent. The computer senses the generated voltage every time the wheel rotates, and from this it can compute speed, distance and start it's internal stopwatch.
This type of computer is very accurate if set up correctly, and has found favour with motorcyclists... find out why, here. The unit pictured has been on the bike for 3 and a half years, as my previous one was stolen... thus the total mileage shown (29,524 miles!) is down by about 1500 miles...
