Sept. 2G, 1884.1 



KNOWLEDGE - 



251 



than did a magnet wound as shown in Fig. 1, the quantity 

 of wire and of iron being in each case identical. For this 

 reason, then, even if no other were available, wire is not 



Fig. 3. 



wound over the bend of the core. This applies almost as 

 forcibly when the wire is wound evenly over the two ends 

 of the core, the centre remaining bare, as shown in Fig. 4 

 (where A repre.^ents the soft iron armature). Another, 

 although, perhap.i, a less important reason for not covering 

 the centre of the bent core is the difficulty that would be 

 experienced in winding, on account of the larger surface on 

 the outside as compared with the inside of the bend, a 

 difference which would obviously increase with every addi- 

 tional layer of wire. 



Fig. i. 



Unless the armature is particularly small, the two 

 "legs" of the electro-magnet are rarely bent so as to 

 make the poles approach each other, as in the case of per- 

 manent steel magnets. They are simply bent parallel, 

 great care being taken to ensure the greatest degree of 

 softness. The core of an electro-magnet should never be 

 of steel, or even of hard cast-iron, because of the much 

 lower magnetic capacity these materials possess as com- 

 pared with good, soft iron. In hammering a piece of iron 

 rod to form the bent core, there is a strong tendency to 

 hardening, and as little hammering as possible should be 

 resorted to, the bend being more efficiently produced by 

 heating the iron to redness, when the metal becomes, 

 naturally, much more workable. In any case, after the 

 proper shape has been attained, the iron should be heated 

 to redness, embedded in charcoal or placed in a clear fire, 

 and then withdrawn and allowed to cool gradually, or, better 

 still, left in the fire until it dies right out. The greatest 

 efficiency at short distances is produced when the wire is 

 coned towards the extremities — in fact, when the magnet 

 has the appearance of being a bent form of that shown 

 in Fig. 3. 



As stated above, electro-magnets are not often called into 

 requisition when only a single pole is required. When 

 fuch a magnet, however, is necessary, it is well to use a 

 long core, winding the given quantity of wire over one end 

 of it, as shown in Fig. -5. The reason for this is, that a 



Fig. 5. 



greater magnetic concentration is produced, or, in other 

 words, the poles are separated by a greater distance, and the 

 one to be used is therefore counteracted to a much less 

 degree by the more distant opposite pole. It must not be 



forgotten that while the one end is inducing one condition 

 in a neighbouring piece of iron, the opposite end is pro- 

 ducing exactly ihe opposite state of affairs, and the result- 

 ing coiidition of the iron is an expression of the difference 

 between the two inductive tendenti^s. 



This fact may easily be demonstrated by pas.sing a current 

 throufih a coil of wire over, say, a thin 6-in. rod of irot), 

 and after noting the stiength, ])lace the coil over the small 

 end of a poker and again pasi-ing the current through it. 

 If the quality of the iron and other conditions are identical, 

 the i)oker will fhow a much greater etiect than the short 

 piece of iron. A coil or bobbin of wire for these esperi 

 meiits may he easily made. Procure a slieet of rather stout 

 foolscap or other paper, and laying it on a hard, smooth sui 

 face, such as that of a drawing-Hoard, place a ruler or other 

 cylindrical rod about three quarters of an inch in diameter 

 across one end of it. Then r^'ll the paper tightly round 

 the rod once, and g'lm thoroughly the remainder of the 

 upper surface of the paper. Next loll the whole sheet over 

 the rod, squeezing as much gum as possible into the porf^^• 

 of tiie pap^r, and forcing out any that may remain unab- 

 sorbed. When the sheet is fully wound, secure it with 

 string, and place it on one side for an hour or 

 two, letting it dry thoroughly. When as-ured 

 that it is dry, press two cardboard or thin wooden 

 discs, about three inches in diameter, over the ends of the 

 paper cylinder. 1 have found cardboard answer very well. 

 It may be m^de rigid by binding a little string round the 

 cylinder, outside the cardboard, and then saturating the 

 string more or less completely with gum, wax, ic. When thi^ 

 is done, remove the string which was used to hold the papev 

 together, and withdraw the ruler. The result will be a 

 thin but very rigid tube, with an excellently smooth inner 

 surface. The space between the discs or cheeks may neat 

 be filled with five or six layers of stout, cotton-covered, 

 copper wire, care being taken to insure that the winding is 

 continuous, as pieviously pointed out. The winding should 

 be done as carefully and evenly as possible. Kinks and 

 bends in copper wire may be readily removed. If allowed 

 to remain they give to the coil an unsightly appearance, 

 besides uselessly taking up valuable space. Of course, the 

 two ends of the wire should be brought out. The 

 inner one may be passed through a small hole in one ef 

 the cheeks near the cylinder. When finished, soak the 

 coil thori'Ughly, in melted paraffin wax to insure insulation, 

 to maintain rigidity, and to exclude moisture. The appear- 

 ance of the coil may be improved by painting it with 

 sealing-wax varnish, which may be made by dissolving 

 some shellac in methylated spirit, and adding a very small 

 quantity of vermilion to the solution. For the present 

 purpose the varnish may be made by di-solving a little 

 ordinary sealing-wax in the spirit, although the common 

 wax is not to be so highly commended for insulating 

 purposes. 



Such electro-magnets as the one illustrated in Fig. 4 are 

 almost invariably adopted for electric bells and othei- 

 similar purposes ; but in telegraphic and kindred apparatus 

 another plan is resorted to, a reference to which we must 

 defer for a fortnight. 



Melted Lead in the Ete. — A ci;rions case of accident from & 

 fragment of melted lead solidifying on tlie surface of the eye 

 without injuring it was recently brought before the Bordeaux 

 Society of Anatomy and Physiology by Dr. Perrier, who showed 

 that the immunity of the eye from burning was really due to the 

 " spheroidal state." The melted jet of lead was at a higher 

 temper.iture than 171" Cent., the temperatu'e necessary to pro- 

 duce the spheroidal state; hence, when it arrived at the surface of 

 the eye it vaporised the moisture of the latter. When it had cooled 

 below 171° Cent, the lachrymal secretion prevented the metal from 

 scorching the ball. 



