Intelligence and Miscellaneous Articles. 245 



i ach of the tubes an insulated wire of copper is wound, always in 

 the same direction, the thickness of the layer of wire being greater 

 on the outermost tube. The extremities of the wire belonging to 

 each tube traverse the breech and are united so as to form only 

 one single conductor arranged in the following manner : the wire, 

 after being wound round one of the two outside tubes, passes on to 

 the inner tube next to this, theu upon the tube concentric to the 

 preceding, and so on to the central tube of this core ; then, having 

 ran along the breech, it is wound round the central tube of the 

 second core, within which core it follows a course the reverse of 

 that indicated for the first core ; that is to say, after winding round 

 each of the concentric tubes successively and in the same direction, 

 passing from the smallest to the largest, it issues finally after en- 

 veloping the outside tube of the second core*. 



The following is the result of some experiments made with this 

 electromagnet : — 



Employing the current of ten Bunsen elements of the usual 

 size with bichromate of potass, the attractive force of the electro- 

 magnet at a distance of 12| millims. is 713 kilogrammes, and the 

 time required for the development of the magnetism to raise this 

 weight is 1*33 second. 



If we cut the wires which pass from one core to the other, then 

 connect the four free ends crosswise (that is, connect the lower end 

 of the wire of the right core with the upper end of the wire of the 

 left core, and the upper end of the wire of the right core with the 

 lower end of that of the left core), and then pass the current of the 

 same ten elements, but combined in two parallel series of five ele- 

 ments each, the power of the electromagnet is unchanged, but the 

 time necessary for the magnetization is reduced to one fourth, or 

 to 0-33 second. 



If, finally, each of the two coils of the electromagnet be covered 

 with a round shield of soft iron, thus binding the upper part of 

 the four concentric cores which constitute it, the electromagnet loses 

 some of its power, and is reduced to the condition of an ordinary 

 electromagnet with a solid core. 



After indicating the theoretical considerations which led to 

 the arrangement adopted, the author continues : — 



" Experiment has shown that, if we cover the polar extremities 

 of the tubes constituting each core with a round shield of iron, the 

 electromagnet loses its great power and is again in the same con- 

 dition as an ordinary electromagnet. In fact, the magnetism re- 

 ceived by the shields will have been developed by the influence of 



* The diameter of the outside tube is \2h centims., and the thickness of 

 the concentric tubes, which are four in number, is about 6 millims. ; the in- 

 sulated copper wire is 1*8 inillim. in diameter ; and the number of helices of 

 wound wire is seven to the outermost, and only two between each of the 

 series of concentric tubes forming a core. In these conditions, the height 

 of the cores being 20 centims., the total length of the wire is about 600 

 metres; its weight, corresponding to this length, is 11*5 kilogrammes ; and 

 the total number of turns is 2000. 



