Ball bearing and rolling cylinder motors

The speed-current characteristic for electrically conducting cylinders has been measured while rolling on parallel rails, current flowing from rail to rail through the cylinders. The characteristic is similar to that of the ball bearing motor. A heavy load can be carried by the rolling cylinders, but there is a minimum current below which the rollers come to a standstill. In the case of stainless steel rollers the load increases with this minimum current at the rate of 2.8 kg/A. However, for carbon rollers the load is proportional to the square of the current, as predicted by electromagnetic theory, rising at the rate of 1.8/spl times/ 10/sup -2/ kg/A/sup 2/. As an alternative to the electromagnetic theory, a model based upon thermal expansion is also discussed.