Now that we know the intensity of the Einstein effect we can say with certainty that, if only his instrumental means had been more refined. Maxwell's experiment would have had a positive result. We can evaluate the magnitude of the effect and we can also calculate that the force which a magnetic needle experiences on account of its internal motions and of the rotation of the earth is thousands of millions of times smaller than the force due to the earth's magnetic field, so that you need not fear from this cause any error in measurements in terrestrial magnetism. In the experiments just discussed we were concerned with forces acting on the material bodies. Maxwell next considers cases in which, always on account of the terms in question, not the material system but the electricity contained in it is set in motion. Here he reverts again to the suspended circular coil, and he points out that when a rotation is suddenly imparted to it there could be produced a transient electric current. Similarly, there would be a current, but now in the opposite direction, when the motion of the coil is stopped. A very simple experiment may serve to give you an idea of these phenomena. We take a cylindrical tumbler, partly filled with water, and set it suddenly in rotation about its vertical axis. Then the friction between the glass and the water will make the fluid rotate likewise, but this will take some time; during a certain period the water will lag behind. Let us next suppose that the motion of the vessel has been kept constant for a sufficient length of time, so that the water has acquired the full angular velocity and then let the vessel be suddenly brought to rest. It is clear that the circulation of the water will continue for a certain time, until it is exhausted by the friction.