Now let a current be made to pass through the coil in the direction N.E.S.W. If electricity were a fluid like water, flowing along the wire, then, at the moment of starting the current, and as long as its velocity is increasing, a force would require to be supplied to produce the angular momentum of the fluid in passing round the coil, and as this must be supplied by the elasticity of the suspending wire, the coil would at first rotate in the opposite direction or W.S.E.N., and this would be detected by means of the mirror. On stopping the current there would be another movement of the mirror, this time in the same direction as that of the current. It does not appear that Maxwell actually tried the experiment; he only says: "no phenomenon of this kind has yet been observed." Now, if for Maxwell's coil we substitute a rod of iron, the magnetization and demagnetization of which are comparable to the starting and stopping of a current in the coil, we have exactly the Richardson-Einstein-de Haas effect that was really observed by Einstein and de Haas and by some other physicists. You know that it amounts to this, that a cylindrical rod of iron suspended in a vertical direction is set rotating, with a sudden jerk, when it is rapidly magnetized or demagnetized. When the magnetization is periodically reversed, the rod is made to oscillate and the amplitude of this motion may be increased by adjusting the frequency of the reversals to that of the free oscillations of the rod. Whereas Maxwell seems not to have tried the above experiment with the coil, he tried to observe another effect. A coil, through which a current could be passed and which could be provided with an iron core, was placed in a system rapidly revolving about a vertical axis, the arrangement being such that the coil was free to rotate in the revolving system, so that the axis of the coil could be inclined to different degrees with respect to the vertical axis of rotation. If in the formula for the kinetic energy there were terms of the kind which Maxwell wanted to detect, there would be a tendency for the coil to place itself with its axis parallel to the axis of rotation; it would behave as a gyroscope and might be called an electromagnetic gyroscope. No trace of a phenomenon of this kind could, however, be observed. I may insert the remark that, if the tendency of which I spoke existed to an appreciable extent, a magnetic needle would, even in the absence of the terrestrial magnetic field, still take a definite position that would be determined by the rotation of the earth; in fact, in virtue of its internal motions the magnetic needle would be comparable to a gyroscopic compass, such as has lately come into use in navigation.