Eilectro-Magnetic Apparatus and Experiments. 255 



Secondly. — Insulate the metallic casing from the magnet, and 

 divide it throughout its length, so that the secondaries cannot 

 pass, and the coil wire will now exhibit the full power of the 

 secondary. 



Thirdly. — Surround an electro-magnet with an entire metallic 

 casing, exterior to the coil wires, and the secondary of the wires 

 will be depreciated as before. Split the casing as before, and 

 the secondary will again have full power. 



Fourthly. — Brass rings or straps surrounding the poles of mag- 

 nets or armatures for magneto-electric experiments, detract from 

 their value by the action of closed circuits. 



Fifthly. — The brass cheeks which are frequently used upon the 

 armatures of magneto-electric machines for supporting the coil 

 wires, materially impair the power of such machines. These 

 cheeks should in all cases be made of wood, ivory, or some non- 

 conducting substance. 



Sixthly. — A metallic casing which entirely envelops a U 

 magnet or armature, cannot convey closed^circuits, as each half 

 of the casing would transmit currents in opposite directions. Con- 

 sequently, (as I have proved by repeated experiments,) the secon- 

 dary of the coiled wire is not in the least impaired by this ar- 

 rangement. - 



The following experiments were tried with a view to ascertain 

 if electrical currents were excited in the body of the magnet it- 

 self A hollow magnet was wound and tried ; the secondary 

 current was not so great as that from a solid bar of the same di- 

 ameter. Singular as it might at first sight appear, the insertion 

 or filling up of this hollow magnet with a rod of soft iron or a 

 bundle of iron wires, did not in the least exalt the force of the 

 secondary. This result accords exactly with that of a similar 

 experiment by Mr. BachoffneT. I then rolled upon a cylinder of 

 wood a piece of sheet iron, not permitting its edges to meet. It 

 was then surrounded with three layers of coiled wire and tried, 

 and the augmentation of the secondary was greater than that 

 produced by the entire hollow magnet, which was of much 

 thicker metal. But when the cylinder of wood was withdrawn, 

 and its place supplied with a bundle of fine iron wires, the sec- 

 ondary was increased to a very great degree, and the whole ap- 

 peared to be equally powerful with a compound magnet of the 

 same size. It should be observed particularly, that when the 



