To measure g by use of a millisecond timer
In this experiment the time taken by a brass ball to fall through a measured height is found by means of an electric clock which measures time intervals in thousandths of a second or milliseconds (ms). This works on a different principle from an ordinary electric clock. It generates electric impulses at the rate of 1000 per second, which are recorded on a counter (Fig. 3.10). Sockets are provided to which start and stop switches may be connected and these are operated by a brass ball at the beginning and end of its free fall through a measured height.
Fig. 3.9 shows the method. The brass ball, supported by hand from a piece of cotton, is held so that it bridges two metal contacts, C. This acts as a stop switch by shorting out the counter, thus preventing it from receiving impulses from the generator. The counter is set to zero and the ball is released. At the moment of release, contact is broken, and the counter starts to record milliseconds. At the end of its fall, the ball strikes a hinged platform, thereby operating a switch which cuts off the impulses from generator to counter. The time of fall ( (in seconds) for a series of different heights x (in metres) is measured in this way and the results recorded in a table as shown. From these a grap is plotted of x against (2, and its gradient, ~ is measured. t As we saw in the previous section, a body starting from rest and falling freely under the action of gravity falls a distance x in time ( given by