310 Royal Society. 



number ; so that the amount of deflection, within certain limits, in- 

 dicates directly, nearly the proportion of electricity which has passed 

 as a current through the instrument. 



If a wire be formed into a square of 12 inches in the side, and 

 then fixed on an axis passing across the middle parallel to two of its 

 sides, and if, when that axis is perpendicular to the line of dip, the 

 whole is rotated, then two of the sides of the rectangle will, in one 

 revolution, twice intersect the lines of force of the earth passing 

 across or through one square foot of area. The currents then tend- 

 ing to move in the upper and lower parts of the rectangle, will con- 

 join to urge one current through the wire ; and if this wire be cut 

 at one place close to the axis, and be there connected with a com- 

 mutator of simple construction, which is described in the paper, the 

 currents round the rectangle may be conveyed away to the galva- 

 nometer, and there measured. Such a rectangle, constructed of cop- 

 per wire one- twentieth of an inch in thickness, gave a certain arc of 

 swing for one revolution. If five or ten revolutions were made, 

 within the time of vibrating of the needle, nearly five or ten times 

 this amount of deflection was produced : the mean result, in the pre- 

 sent case, was 2°"624 per revolution. When the same length of the 

 same wire was arranged in oblong or oblate rectangles, so as to 

 diminish the inclosed area in different directions as regarded the axis 

 of revolution, still the deflection was in every case proportional to 

 the areas included ; showing that the effect produced was propor- 

 tional to the number of lines of force intersected by the moving wire. 

 The same result was obtained when two squares having areas in the 

 proportion of 1 to 9, were employed. 



When squares of the same area were formed of copper wire of 

 different thicknesses, then the effects of obstruction in the conducting 

 part of the system were brought out and measured. Thus, with 

 wires which were 005, 0T and 02 of an inch in diameter, and 

 therefore in mass as 1, 4, and 16, the deflections were 1, 2"78, and 

 3 - 45 ; a result almost identical with that obtained for the same wires 

 by the use of loops and a local magnet in the former researches. 

 When two equal rectangles were compared, one containing a single 

 circuit of 4 feet of wire 04 in thickness, and the other four circuits 

 of 16 feet of wire 005 in thickness, then the first was found to 

 evolve the largest quantity of electricity ; but the second, electricity 

 of the highest tension, by the same amount of motion : the accord- 

 ance of these results with the principles advanced is pointed out. 

 The author then refers to the use of wire rings of one or many con- 

 volutions, and indicates cases in which they may supply valuable 

 means of experimental inquiry. 



The relative amount and disposition of the forces of a magnet 

 when it is alone, or associated with other magnets, forms the next 

 point in the present paper; and a distinction is first taken between 

 ordinary magnets, which are influenced much by other magnets, so 

 that the amount of their external force varies greatly, and those 

 which are very hard, where this influence is reduced to little or 

 nothing. The power of a given magnet was measured according to 

 the method described in the last series, by a loop once passed over 



