MEASURING THE SPEED OF DIGITAL SIGNALS (SPEED OF LIGHT)

An odd number of digital not-gates connected in a closed loop circuit will produce a relatively stable free running oscillator, The frequenc y of oscillation is determined by the number and type of not-gates, th e circuit supply voltage, and the total length of connection wire used between the not-gates. Measuring the frequency for various lengths of connection wire enables the speed of the digital signals in the wire to be calculated. The method is sensitive enough to reveal that signal s traveling in 22 gauge insulated copper wire propagate about 5% slowe r than those in 22 gauge bare copper wire. The speed in insulated wire is (2.87+/-0.03)x10(8) m/s and the speed in bare wire is (3.03+/-0.02 )x10(8) m/s. This experiment has been found to be a very popular under graduate laboratory due to its low cost, straight forward theory, and relatively high (similar to 1%) precision. The absence of a light sour ce eliminates complicated optics and impresses students with the fact that visible light is not the only thing that travels with the speed o f light! (C) 1996 American Association of Physics Teachers.