XVII. An Atomic Model. By R. R. Ramsey, Ph.D., Associate Professor of Physics, Indiana University, Bloomington, Ind., U.S.A. [Communicated by the Author.] MODERN theories of the structure of an atom assume one or more electrons in motion about a central body or positive nucleus. Probably the experiment which has been the most helpful in giving an idea as to the structure of an atom is the Mayer [citation redacted] experiment of the floating needles. This, together with the work of J. J. Thomson [citation redacted], has become almost classic. The experiment gives an idea of the possible structure of atoms, and may account for the periodic variations of the properties of the atoms. Thus one, by assuming that an atom of large atomic weight has more electrons than one of 208 [header] small atomic weight, may account for the periodic table, as has been done by Lyon. [citation redacted] The classical method of performing the experiment is by floating magnetized needles by means of corks in water. I have found that small bicycle-balls floated on mercury are much more convenient. (Professor Merritt used this method at Cornell University in 1900.) The mercury surface lends itself admirably for projection with reflected light. In projection it is well to focus not on the balls, but on a plane a short distance above the balls, or on the focal point of the concave mirror made by the depression caused by the balls. The position of the ball is then shown on the screen as a point of light. In the classical Mayer experiment the balls are fixed. There is nothing to suggest how the atom may radiate. The atom is dead. The motion of the atom must be imagined. It is usual to imagine the needles to rotate about the centre with a constant angular velocity. This is contrary to the laws of planetary motion as illustrated in the solar system. While working with this experiment the thought came to me to rotate the mercury, and thus to rotate the balls. A wooden tray was made with an electrode at the centre and four electrodes, one at each corner, which were connected in multiple. By sending the current in at the middle electrode and out at the corners one has an approximately radial current flowing at right angles to the magnetic field of the magnet, which plays the part of the positive nucleus in the experiment. This causes the mercury to rotate and carry the balls with it. The apparatus consists of a wooden tray, as shown in fig. 1. The dimensions are 15 x 15 cm. and 2 cm. in depth. The electrodes C and M are made of platinum. (It has been found later that the electrodes M can be made of iron without appreciably distorting the magnetic field.) A and B are binding posts which are connected to the electrodes by wires, shown by dotted lines, which are in grooves on the under side of the box. The apparatus can be centred up by placing one bicycle-ball on the mercury surface after the current has been turned on through both the magnet and the tray, and then shifting the tray until the ball remains practically stationary at the centre of the rotating mercury. When two balls are placed on the rotating surface they do not rotate about the centre on the same circle, as one would expect from the Mayer experiment. No. 1 first rotates about No. 2, and then No. 2 rotates about No. 1. Their paths resembling rotating ellipses. With three balls the motion is [header] 209 more complicated, the three balls taking turns in the centre. The motion reminds one of a complicated game of leap-frog. With a number of balls the motion becomes very complicated. [figure redacted] The mercury at the edges of the box is stationary while the central portion is rotating. The angular velocity increases as we go from the edge to the centre. The balls floating on the surface tend to take up the same angular velocity as the mercury upon which they float. There is a tendency for the balls to take up a motion which may approximate to planetary motion. Thus we may assume that they obey Kepler's law. In the Mayer experiment, when the balls are stationary, when there are a number of rings any one ball is held in its place by the central force of the magnet and the mutual repulsion of the neighbouring balls. The balls of one ring fit into the crotches of the neighbouring rings. When the balls are not stationary and the angular velocity of the outer ring is less than that of the inner ring, there is a slipping of one ring with respect to the one next to it. This slipping produces a perturbation or a vibratory motion which is superimposed on the regular circular motion. This perturbation may be said to be the source of some sort of radiation, light perhaps. When a ball from the outside is allowed to come into the system there is a great disturbance of the whole system. If the balls represent electrons, this disturbance may be said to be the source of the X rays, as when a cathode ray hits a platinum atom, say. [footer] 210 [header] With a large number of balls the motion is very much more complicated than one would expect. At times a ball will start out from the outer ring and apparently seem to try to escape from the system. In consequence of the friction of the mercury and the nature of the field, the ball always returns. If a ball were to escape it would cause a rearrangement of the others, or a disturbance similar to that caused by an added ball. This tendency of the ball to fly off is especially great if the current through the mercury is increased, or if the system absorbs energy. This may be an illustration of what takes place in the photo-electric effect, or in the case of ionization produced by hot bodies. In the case when a ball flies out when rotating at normal or constant velocity, we have an explanation of [gamma] rays caused by [beta] rays. Or we may let the balls represent [alpha] rays, helium atoms, or that which in the atom of radium makes [alpha] rays or helium atoms after they have escaped, and we have an illustration of a radioactive substance. To illustrate the disintegration of an atom of radium through its several disintegration products, I made a tray in which I embedded a ring of iron, so as to make a magnetic field which is strong at the centre and diminishes as we go along any radius, passing through a minimum and then [figure redacted] through a maximum over the ring of iron. Fig. 2 is a cross-section of the tray and central magnet. N S is the central magnet. R R is the cross-section of the iron ring. A and B are binding posts by which the current is led in [header] 211 and out. The variation of the field is represented by lines of force. To use this the current is turned on the magnet and a number of balls are placed in the centre of the tray, forming the characteristic figure due to the particular number as in the Mayer experiment. The current is then turned through the tray causing the balls to rotate. When a ball at irregular intervals starts out on a tangent, it will be caught and held by the intense field over the iron ring at R. Thus if the ball represents an [alpha] particle, the escape of [beta] rays and the [gamma] radiation may be explained as being due to the disturbance in the atom due to the rearrangement in the atom. As many as eight or ten balls may escape from the system, each rearrangement of the system representing one of the products in the radioactive series. Getting the conditions right is a matter of trial. Some three or four trays were made before one was satisfactory. The dimensions of this tray are as follows : length 10 cm., breadth 10 cm., depth 2 cm. The iron ring is made of a 21/2 millimetre rod bent into a ring of 6 cm. diameter. No doubt many analogies will occur to the operator which have not been mentioned in this paper. The worst difficulty with the experiment is with the mercury. The mercury must be clean. Any film of dirt or dross on the surface of the mercury prevents the free motion of the balls. The magnet and tray maybe connected in series, but it is more convenient to have two circuits which may be manipulated independently. Dept. of Physics, Indiana University. Nov. 7th, 1916.