266 laaa aan Apparatus and. E'xperiments. 
magnet, anit that point most remote from the’ point of ‘contact. — 
These experiments throw some light upon a fact which, though 
Jong since known, does not seem to have been understood ; viz. 
an armature which entirely subtends the poles of a U magnet, will 
hot sustain so great a weight as one which covers only about one 
third of each pole. If the surface of the armature be flat, it will 
not be held so firmly as if spherical, presenting much fewer 
points. If the armature be flat and broad, that portion over .the 
pole may be considered inthe light of the soft iron disc. Nu- 
merous holes in an armature do not sensibly interfere with its 
adhesion. A piece of soft iron was first suspended from a single 
pole, with just'as much weight as it would hold. It then had 
several large holes drilled through it, taking away a large portion 
of its substance, and was again tried; the induced magnetic 
power appeared to be.as great as thenagh the entire piece. ‘This 
doubtless would not be true to any extent, although the proper- 
ties of the armature are not perceptibly affected by a hole through 
its center, yet if a steel, or soft iron rod, be passed through. this 
hole, its inductibility will be greatly impaired. This fact should 
be particularly observed in the construction of magneto-electric, 
and electro-magnetic. machines, where a steel, or iron shaft, is 
often allowed to pass through an armature or magnet. If, while 
the armature is suspended by one end to a single pole, a piece of 
soft steel is drawn through the hole in its center, the steel be- 
comes properly and permanently polarized ; but if, while the ar- 
mature is thus in contact with the magnet, the steel rod be passed 
half its length through the hole, and Seiaraiued j in that situation, 
both its extremities will be found to be similar poles. 
In the management of electro-magnetic engines, it is worth 
observing here, that a greater power is always obtained by using 
a compound, instead of a Single battery, provided the series does 
not exceed.‘two. As the elementary battery has always been 
considered as possessing the greatest dynamic, ot magnetic power; 
this species of battery has been preferred for application to elec- 
tro-magnetic machines. I-have invariably found that two pairs 
of plates, arranged as a compound series, connected with an elec- 
__ tro-magnetic engine, or any apparatus for electro-magnetic rota- 
tions, produce a velocity nearly double that given by the same 
surface used as an elementary battery. If the series extend be- 
yond two, the magnetizing pare diminishes, although the a 
