198 Mr J. D. FORBES on eliciting an Electric Spark 



interesting an experimental truth, though not, if report tells true, 

 until after Signer NOB i LI, the ingenious and persevering philo- 

 sopher of Reggio, I have taken advantage of this last meeting 

 of the Society for the season, to bring the subject before them : 

 and though I regret that, from the nature of the experiment, it 

 can only be shewn to one or two persons at once, I believe that 

 there are several individuals now present, who can bear ocular 

 testimony to the production of the spark. 



The discovery of Mr FARADAY has conclusively demonstra- 

 ted, that in every case where a magnetic current is created (to 

 use the word current in its ordinary acceptation, as indicative of 

 a peculiar condition, and without reference to any theory what- 

 ever), a momentary electric current is induced at right angles to 

 it. The experiment may be shewn in two ways : either by me- 

 chanically causing a magnetic bar to traverse the axis of a helix 

 of copper-wire of considerable length, or by causing a piece of 

 soft iron, placed in the axis of such a helix, to connect the poles 

 of a horse-shoe magnet, and thus temporarily acquire polarity. In 

 both cases, the current of electricity is most easily shewn by its 

 action on the common Multiplier, the extremities of the wires 

 of which are connected with those of the copper- wire forming 

 the helix, and which, from its rectangular position at every point 

 to the magnetic current caused to traverse its axis, collects the 

 electricity developed in that direction. By both methods, the 

 current is only instantaneous ; and in the first case, if the mag- 

 net be caused to make a reciprocating motion in the axis of the 

 helix, the currents are of course excited in opposite directions, 

 and, by accommodating these to the alternate vibrations of the 

 needle of the Multiplier, it 'may, by means of a small bar, be 

 made to oscillate in large arcs. 



The second method, however, where the magnetic current 

 is created through a piece of soft iron, by causing it to connect 

 the poles of a large magnet, is that which in my late experiments 



