It's designed to be a low-cost but accurate short-interval timer, suitable for a whole range of purposes (not only reaction). There is no case (cost saving #1), the push-button switches are mounted in old film cannisters (or anything else you wish - cost saving #2) and there is no output circuitry or display, because the output is read directly on any digital multimeter - cost saving #3.

There's the short time for the nerve impulses from your senses to travel to your brain, the time for your brain to respond and then a further short time for outgoing nerve impulses to travel to your limbs and stimulate the muscles to produce your reaction.

These three delays or 'latency times' are usually lumped together into a single quantity known as your reaction time: the total time taken for you to actually respond to such a stimulus.

Your reaction time varies depending on whether you respond with your hand or your foot. It also depends on your state of health, alertness, psychological outlook and whether you have recently taken drugs or alcohol.

The reaction time for a normal healthy adult seems to vary between about 150 and 300ms (milliseconds) for a hand response and between 400 and 800ms for a foot response (eg, hitting the brakes). If you are driving a vehicle and your measured reaction times are significantly longer than these times, you are an "accident waiting to happen".

You don't need to be a rocket scientist to work out why. Consider driving at 70km/h. At that speed, you're travelling a distance of 19.4 metres every second or almost two metres in each 100ms. So if it takes you (say) 500ms to respond to an emergency by stepping on the brake pedal, your car will travel almost ten metres before the brakes can even begin to slow you down.

Some safety experts have been lobbying for years to make reaction time testing mandatory for driver's licence renewals. It hasn't happened yet - but in the meantime you can measure the reaction time of all your driving friends, to judge whether they should be on the road or not...