Note on Mr. Wood's Method of Illustrating Planetary Orbits

Abstract

I FEAR that Mr. Wood's beautiful method of illustrating planetary orbits by means of a bicycle ball rolling on a glass plate about the pole of an electro-magnet (NATURE, April 29, 1897), has rather fallen into disrepute in the minds of many physicists since its criticism by Mr. Anderson in NATURE, May 13, 1897. Mr. Anderson there states that the law of attraction in such a case would be that of the inverse fifth power of the distance. This could only be true if the ball were of very soft iron. A bicycle ball is far from this, and becomes strongly magnetised after brief use in the experiment, behaving like a permanent magnet of great coercive force. Under these conditions the attraction between the pole and the ball will vary approximately as the inverse third power. There is also another factor to be considered. If the true pole lies below the glass plate, only a certain component of the total force is active in producing the attraction towards the centre of motion. To determine what the law of variation of this component will be, I have had one of my students take a number of series of observations on the attraction of a bicycle ball along a plane perpendicular to the axis of a magnet.