324: MAGNETO-ELECTRIC APPARATUS. SECT. XXXII. 



Voltaic electricity alone is too feeble to overcome the resistance 

 of a long wire. 



Electric currents, whatever their tension may be, produce the 

 phenomena of induction ; these again induce other currents in bodies 

 capable of induction, and so on indefinitely ; the first and second 

 flow in the same direction, the others alternately opposite and 

 direct. They all give the shock and can decompose water, but 

 with Volta-electric currents the elevation of temperature as well 

 as their physiological and magnetic effects are produced by inr 

 stantaneous actions, which only depend upon the quantity and 

 tension of the current, and by no means on its duration, for in- 

 duced currents only exist for a moment when the circuit of the 

 battery is broken. The most energetic physiological eifects are 

 produced by a small quantity of electricity moving with great 

 velocity. The apparatus first employed by Dr. Faraday is in 

 effect a battery, where the agent is the magnetic instead of 

 the Voltaic force, or, in other words, electricity, and is thus 

 constructed : 



A very powerful horseshoe magnet, formed of twelve steel 

 plates in close approximation, is placed in a horizontal position. 

 An armature, consisting of a bar of the purest soft iron, has 

 each of its ends bent at right angles, so that the faces of those 

 ends may be brought directly opposite and close to the poles of 

 the magnet when required. Ten copper wires covered with 

 silk, in order to insulate them are wound round one half of the 

 bar of soft iron, as a compound helix : ten other wires, also insu- 

 lated, are wound round the other half of the bar. The extremi- 

 ties of the first set of wires are in metallic connexion with a 

 circular disc, which dips into a cup of mercury, while the ends 

 of the other ten wires in the opposite direction are soldered to a 

 projecting screw-piece, which carries a slip of copper with two 

 opposite points. The steel magnet is stationary ; but when the 

 armature, together with its appendages, is made 'to rotate verti- 

 cally, the edge of the disc always remains immersed in the 

 mercury, while the points of the copper slip alternately dip in it 

 and rise above it. By the ordinary laws of induction, the arma- 

 ture becomes a temporary magnet while its bent ends are oppo- 

 site the poles of the steel magnet, and ceases to be magnetic when 

 they are at right angles to them. It imparts its temporary mag- 

 netism to the helices which concentrate it ; and, while one set 



