This experiment is a variation of the standard Central Scientific Co. apparatus. The rotating mass is mounted on a horizontal board which is about 14 inches long and eight inches wide. The mass is a flat plate which is mounted on three ball bearings which rest in V-grooves in the plate attached to the board. A spring with adjustable tension pulls this mass towards the axis of rotation. Two flashlight batteries and lamps are also mounted on the bo&rd. The lamps are on the axis of rotation. Contacts at the inner and outer edge of the plate which support the mass light these two lamps. When the speed of rotation or the tension is adjusted so that neither of these lamps are lit, then the tension in the spring is supplying the centripetal force needed to accelerate the mass in its circular motion.