88 Electro-Magnetic Apparatus. 
which is essential, and the only advantage of the spiral direction is to 
keep the current for a longer time, in the vicinit¥ of the needle. 
lieving that the great advantages gained by Prof. Henry, in multiply- 
ing the wires and diminishing their length, were almost wholly resol- 
vable into the superior effect of a large current compared with a small 
one, I have been induced to seek for similar advantages by a simpler 
method, by which moreover the resultant of the force is exactly 
transverse, and the current on arriving at the magnet, undergoes a 
sudden dilatation, and a consequent diminution of velocity. Prof. 
Henry, has ingeniously inferred from some of his experiments, that a 
certain degree of slowness may be favorable. This is also consist- 
ent with an opinion expressed in the above quotation. 
Another peculiarity then of this apparatus, is the small dimensions 
of the channel which conducts the electricity to the magnet, in com- 
parison with that in which it circulates around it. It was found by 
trying wires of different sizes and in different numbers, that within — 
wide limits, a reduction in the magnitude of these uniting wires, o¢- 
casioned but a slight diminution of the magnetizing effect. For in- 
stance, with the battery employed, the difference between the effect 
of four bell wires at each end of the sheet, and that of two at each 
end, was scarcely perceptible. Much more.depends upon the length, 
breadth and proximity of the sheet along which the electricity circu- 
lates around the iron. 
The conducting property of the silk fabric, produces a sensible di- 
minution of the effect. Suspecting that in this and other electro-mag- 
netic apparatus, there might be some transverse communication of @ 
part of the electricity, I employed in one instance two layers of thick 
silk instead of one of the same kind, and found the power of the mag 
net to be thereby increased, notwithstanding the removal of the cop- 
per to a greater distance by the interposition of an additional layer of 
silk. It would seem from this fact, that regard should be had to the 
want of a perfectly nen-conducting property in the coating, when we 
attempt to establish some of the laws of electro-magnetic action. This 
would seem to be still more necessary in other apparatus, in which 
coated wires of great length are employed. 
In the apparatus high. has been described, although the electrical 
currents are symmetrical, and the resultant o their forces, exactly at 
right angles to the axis of tle iron cylinder, and consequently in the 
a 
iteaaiaianiaaes neal 
* Vid. American Journal of Science, Vol. x1x, p. 399. 
et 
