Electro-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 - 
seconda 
Thirdly,  Sanoutid an Sidiacotangues writt an entire metallic 
casing, exterior to the coil wires, and the secondary of the wires 
will be depreciated as before.. Split the casing as s before, and 
the secondary will again have full power. —- 
Fourthly. —Brass rings or straps surrounding the aoa of mag- 
nets or armatures for magneto-electric experiments, detract from 
their value by the action of closed circuits. 
Fifthly.—T he 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 candle 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 
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. 
i The following scpelheats 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 frem 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. Bachoffner. 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 But when the cylinder of wood was withdrawn, 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 
