Abstract :
EVERY engineer knows how difficult it is to make accurate speed measurements and how essential accuracy is in certain cases, for instance, in testing and experimental work. In practise, speed counters and tachometers are generally used. The first register the number of revolutions made by the machine in a given time and must be employed in connection with a time measuring device, usually a stop watch. Here the sources of error are slip and the notorious inaccuracies of stop watches. The speed counter should be applied from 30 to 60 seconds, but during that time many changes can take place in the operation of the machine under test. Slip errors can be held down by the exercise of constant care or entirely eliminated by using suitable drives, but such drives cannot always conveniently be employed.
