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 vicinity of the needle."* Be- 

 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, oc- 

 casioned but a slight diminution of the magnetizing effect. For in- 

 stance, with the battery employed, the difference between the effect 

 of four bell vt^ires 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 a 

 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 which has been described, although the electrical 

 currents are symmetrical, and the resultant of their forces, exactly at 

 right angles to the axis of the iron cylinder, and consequently in the 



Yid. American Journal of Science, Vol. xix, p. 399. 



