Ampère's Rule

As Prof. Daehne (NATURE, June 24, p. 168) has called attention again to the treatment of Ampere's rule in my “Electricity Treated Experimentally,” perhaps you will allow me to point out that the rule given by Ampere is quoted historically only, and for it is substituted a rule, due, I believe, to Clerk-Maxwell, which seems to me preferable to either the original rule of Ampere, or to that quoted by Prof. Daehne, namely, that the movement of a north pole is right-handed to the direction of the current. That is to say, if we assume any right-handed screw to be propelled along the current, the north pole will move in the direction of the twist in the muscles of the wrist in propelling it; and vice versâ, if the north pole move in the direction of propulsion, the current urging it will be in the direction of twist in the muscles of the wrist. In treating the movement of a conductor carrying a current in the magnetic field, I have used a rule identical in character with Ampere's, and that was probably the rule to which J. T. B. referred in his critique, namely, that a figure swimming in the current and looking along the lines of force is carried to his left. I should be glad to find a rule at once as complete and more simple, although after a pretty wide experience, not always with the very brightest of pupils, I have not been sorely pressed with the difficulty J. T. B. seems to have felt, All the required attitudes are pretty familiar to a boy who is accustomed to diving in the water and swimming on his front, side, or back.