Magnetic Figures illustrating Electrodynamic Relations. 349 



rangement of magnets or of conducting-wires had been made 

 for the particular case of the experiment, and the plate been 

 laid in a horizontal position, fine filings of wrought iron pre- 

 viously sifted were dusted over the plate through muslin, and 

 the plate was tapped lightly with vertical blows from a piece 

 of thin glass rod. When the filings had arranged themselves, 

 and the plate was still in situ, a gentle current of steam was 

 allowed to play upon the plate, condensing upon the surface of 

 the gum and softening it, and thus allowing the filings to 

 embed themselves where they lay. After the gum had again 

 become hard, the prepared face was covered by a protecting 

 plate of glass, on which in certain cases were drawn the posi- 

 tions of the wires or magnets employed. The figures_fixed in 

 this manner are suitable for projection with the lantern upon 

 the screen. They can be readily photographed for transpa- 

 rencies, or for paper prints ; specimens of each of these 

 methods of photographic reproduction are exhibited to the 

 Society. 



Figure 1 represents the condition of the magnetic field sur- 

 rounding the current in a straight conducting-wire, which was 

 carried vertically through a hole drilled in the plate. The 

 wire employed throughout the series was a silver one of about 

 •8 millim. in diameter. The battery power employed for this 

 experiment was that of 20 Grove's cells arranged in two series 

 of ten each. In some of the succeeding experiments a less 

 current was found sufficient. 



But for the imperfections of the method of experiment, these 

 curves would be perfect circles, and the distances between 

 two successive lines of force would be proportional to the 

 square of the distance from the central point. The equipoten- 

 tial magnetic surfaces, being always normal to the magnetic 

 lines of force, would be represented by a system of radial lines 

 forming equal angles with one another. There appears to be 

 no recognized name for the closed curves traced out by the 

 lines of force around conductors carrying currents. With 

 great diffidence I therefore beg to speak of them as isodynamic 

 lines. They are theoretically disposed about a single straight 

 conductor in a perfectly concentric manner, and at such dis- 

 tances apart as would be defined by the requirement that a 

 parallel conductor, carrying a like current of unit strength, 

 would do unit work in passing from one isodynamic line to the 

 next. The absolute value of an isodynamic line would of course 

 be determined (like magnetic and electrostatic potential) by 

 the work done by a like element of current in passing to any 

 point in that line from an infinite distance. No work is done 

 in moving an element of a parallel current along an isodyna- 



