The Atwood machine is a fairly standard version with two fifty-gram weights connected by a thread passing over a light pulley. A catch with a quick release can hold one of the weights at the upper end of its motion. If a five to ten gram weight is placed on one of the fifty-gram weights, it will accelerate when the catch is released. A microswitch with a variable position is placed below the weight in such a way that as the weight passes by it, it removes the unbalancing weight. The fifty-gram weight then proceeds at a uniform velocity. Below this microswitch there is a second microswitch. The first microswitch turns on a Hunter counter; the second microswitch turns it off. It is thus possible to measure the uniform velocity which is attained after accelerating a mass of one hundred grams through a given distance with a known force.