266 Electro-Magnetic Apparatus and Experiments. 



magnet, and that point most remote from the point of contact. 

 These experiments throw some light upon a fact which, thongh 

 long since known, does not seem to have been understood ; viz, 

 an armature which entirely subtends the poles of a U magnet, will 

 not sustain so great a Aveight as one which coversonly 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 fliat and broad, that portion over the 

 pole may be considered in the 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 poition 

 of its substance, and was again tried ; the induced magnetic 

 power appeared to be as great as through 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 examined in that situation, 

 both its extremities will be found to be similar poles. 



Ill 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 batter}^, provided the series does 

 not exceed two. As the elementary battery has always been 

 considered as possessing the greatest dynamic, or 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 power diminishes, although the sparks 



