COIL-MACHINES. 59 



further information, to consult the excellent works of Mr. 

 Xoad,* or Mr. C. V. Walker, f on this subject. 



If a copper wire be twisted in the form of a helix, and a 

 current of electricity be passed through it, it induces another 

 current of electricity in any other coil which is in its immediate 

 vicinity. If a small bar of iron, or what is better a bundle of 

 iron wires, be introduced into the axis of the helix, and a 

 current of electricity be passed through the coil of wire, it will 

 be found that the electric spark, and its accompanying snap, are 

 much increased; but it is only on breaking contact with the bat- 

 tery that this effect is produced; the reason is that the iron, 

 magnetised by the power of the continuing current, loses its 

 magnetism at the moment the current ceases to pass, and in so 

 doing tends to produce an electric current in the wire round it. 



Mr. Callan, of Maynooth College, was the first who contrived 

 a convenient apparatus for the illustration of secondary currents. 

 A coil of thick insulated copper bell-wire is wound on a small 

 bobbin ; and on a large rod, with a hollow axis, in which the 

 bobbin may be introduced at pleasure, a length of about 1500 

 feet of thin wire is wound ; the two coils are thus perfectly dis- 

 tinct from each other, and by sending the current from the bat- 

 tery through the interior coil, the Electricity present in the 

 exterior coil is set in motion by its inductive influence ; and from 

 it both physiological and electrolytic effects may be obtained. If 

 100 yards of fine insulated copper wire be wound on a reel, and 

 contact with an electrometer rapidly broken, shocks may be 

 obtained, by grasping metallic cylinders in connection with the 

 ends of the coil, from the reflex wave of Electricity which is 

 generated; and if a bundle of iron wires be placed in the axis 

 of the helix, the brilliancy of the sparks, and the intensity of the 

 shocks will be greatly increased, in consequence of the second 

 wave of Electricity being, as we have seen, produced at the 

 moment of the demagnetising of the iron. 



It is on this latter principle that the greater number of the 

 Electro-Magnetic Coils are constructed. There are many forms 



* Lectures on Electricity, by Henry M. Noad. Lend. 1844. 



t Second volume of Electricity in Lardner's Cabinet Cyclopaedia. 



