and Experimental Researches therewith. 55 



known to myself, I have only to introduce into the opposite 

 coils different coins successively, as I should weights in a 

 scale, and when perfect balance is announced by the silence, 

 the amount in one box will not only be the same nominal 

 value but of the same kind of coin. 



IV. We find, by direct experiment with this instrument, 

 that the preceding results are due to electric currents induced 

 by the primary coil, and that it is by the reaction of these 

 that the balance is destroyed; for if we take an insulated 

 spiral disk or helix of copper wire with its terminal wires 

 open, there is no disturbance of the balance whatever, not- 

 withstanding that we have introduced a comparatively large 

 amount of copper wire ; but on closing the circuit the balance 

 is at once very powerfully disturbed. 



If the spiral is a flat one, resembling a disk of metal, and 

 circuit closed, we find that loud tones result when the spiral 

 is placed flat, or when its wire is parallel to those on the coils; 

 but if it is held at right angles to these wires, no sound what- 

 ever is heard, and the balance remains perfect. The same 

 thing occurs with disks of all non-magnetic metals, and a disk 

 of metal placed perpendicular to the coils exerts no influence 

 whatever. The contrary result takes place with a spiral of 

 iron wire or disk of iron : the induced current circulating in 

 the spiral is at its maximum when the spiral lies flat or parallel 

 with the coils, giving no induced current whatever when at 

 right angles; but the disturbances of the induction-balance are 

 more than fourfold when perpendicular to the wires of the 

 coils than when parallel with the same. This result is simply 

 due to the property of magnetic bodies, of conduction of mag- 

 netism. 



V. If we introduce a disk of metal gradually into the 

 coils, we find that its power increases as the square of the 

 distance, until it arrives at its maximum exactly in the centre 

 of the vacant space between each pair of coils, diminishing 

 rapidly in the same ratio if the disk be moved towards primary 

 or secondary. Thus, in the interior of the coils there is but a 

 single line of maximum force ; but at the exterior we have 

 on each coil two maxima and one minimum, the first and 

 most powerful maximum being in the centre of vacant space 

 between each pair of coils, coinciding with the maximum 

 lines of force of the centre of the coils ; the minimum lines of 

 force are exactly in the exterior centre of each coil, again 

 rapidly rising to a maximum near the exterior edge of coils, 

 and gradually diminishing in power from this point. 



If we place exteriorly a bar of metal in the centre line of 

 vacant space, we find that it has here its maximum disturbing 

 power, giving out loud sounds. 



