illustrating Electvodynamic Relations. 353 



the axis of the magnet. In the figure, where the vertical cur- 

 rent passes upwards through the glass, the neighbouring south- 

 seeking pole (marked in position by a square dot) is urged 

 round the current with a couple tending in a right-handed 

 cyclical rotation. The couple is reversed, and acts in a left- 

 handed order, if either the current or the magnet-pole be 

 reversed. The contrasted dissymmetry so produced is very 

 curious, and the mutual displacement of the radial lines of 

 force of the pole and of the circular lines of the current is very 

 significant. 



Figures 17 and 18 show lines of force arranged spirally in 

 the field. In these a current passes upwards through the glass, 

 while the pole of a magnet is placed vertically beneath : the 

 current, in fact, passes through the magnet. The form of the 

 lines of force is remarkable. No work would be done on or 

 by an element of a vertical current in bringing it up to the 

 centre along one of the spiral lines ; for the work done by it 

 in bringing it in the spiral path across the successive circular 

 isodynamic lines of the current would be equal to that done 

 upon it in carrying it across the successive radial lines of force 

 of the magnet-pole. The equipotential surfaces of this field 

 are consequently another series of spirals of an opposite cy- 

 clical order. In figure 17 the current running up through a 

 south-seeking pole produced a right-handed spiral ; in figure 

 18, with a north-seeking pole the spiral is of the opposite order. 

 Since the magnetic potential decreases from a magnet-pole 

 with the inverse square of the distance, and since the inductive 

 action of the current on a point in the plane of the figure also 

 decreases according to the inverse square of the distance from 

 the current, each branch of the spirals would be described by a 

 moving point whose angular displacement from the arbitrary 

 zero is simply proportional to the distance from the central 

 point. The results of actual measurement of the spirals at 

 successive distances of whole millimetres from the centre show 

 as near an agreement with this supposition as the roughness 

 of the method of procuring the curves permits. The number 

 of branches of the spiral will clearly be proportional to the 

 strength of the magnet pole. The obvious result of a " short- 

 ening " of the spiral lines would be to produce a rotational 

 movement, such as we know to be produced on a free-magnet 

 pole under the influence of a current traversing it longitudi- 

 nally. 



I am indebted to Mr. Robert Gillo, of Bridgwater, for the 

 admirable photographic copies of the various figures. 

 June 19, 1878. 



Phil Mag. S. 5. Vol. 6. No. 38. Nov. 1878. 2 A 



