Masterpieces of Science 



to the other. The same principle was extended 

 by employing a still longer insulated wire, and 

 winding several strata of this over the first, care 

 being taken to insure the insulation between 

 each stratum by a covering of silk ribbon. By 

 this arrangement the rod was surrounded by a 

 compound helix formed of a long wire of many 

 coils, instead of a single helix of a few coils, 

 (Fig. 5)- 



In the arrangement of Arago and Sturgeon the 

 several turns of wire were not precisely at right 

 angles to the axis of the rod, as they should be, 

 to produce the effect required by the theory, 

 but slightly oblique, and therefore each tended 

 to develop a separate magnetism not coincident 

 with the axis of the bar. But in winding the wire 

 over itself, the obliquity of the several turns 

 compensated each other, and the resultant action 

 was at right angles to the bar. The arrange- 

 ment then introduced by myself was superior to 

 those of Arago and Sturgeon, first in the greater 

 multiplicity of turns of wire, and second in the 

 better application of these turns to the develop- 

 ment of magnetism. The power of the instru- 

 ment with the same amount of galvanic force, 

 was by this arrangement several times increased. 



The maximum effect, however, with this ar- 

 rangement and a single battery was not yet ob- 

 tained. After a certain length of wire had been 

 coiled upon the iron, the power diminished with 

 a further increase of the number of turns. This 

 was due to the increased resistance which the 

 28 



