Fig. 1. The Gauss accelerator ready to delight and amaze.

James Rabchuk’s recent paper1 describes amethod for measuring the kinetic energychanges in the Gauss accelerator, as well as a calculation of the change in potential energy. In this paper, a simple method for measuring both the change in potential energy and the change in kinetic energy will be presented. The measurements can be made with rulers, strings, and weights. In the process, your students will learn about the relationship be-tween work and potential energy as well as the law of conservation of energy. Issues associated with the law of conservation of momentum in the accelerator will also be addressed. The Gauss accelerator2 pictured in Fig. 1 consists of an 8-mm diameter spherical neodymium magnet3 and three 5/16-in (0.313-in) steel ball bearings. The device is available with cube- shaped magnets from Science Toys.4 I first saw the Gauss accelerator in a much larger version presented to a local AAPT meet-ing at the Exploratorium, by Paul Doherty of the Teacher Institute. When the lone ball on the left is gently brought in toward the right [Fig. 2(a)], it accelerates until it col-lides with the magnet. The result is that the incoming ball sticks to the magnet, and the ball on the right takes off with a surprisingly large velocity [Fig. 2(b)]. Where Does the Energy Come From? Since the outgoing ball is moving faster than the incoming ball, the kinetic energy of the system has in-creased. According to the law of conservation of ener-gy, the potential energy of the system must have de