UNDULATORf FORCES. ELECTRO-MAGNETISM. [BLKCTRO-MAOHKIS. 



the removal of the wires from the battery, that the 

 metal doe* not lote its magnetism. On the contrary, it 

 hai been converted into a permanent magnet. Herein. 

 therefore, exists a similar difference between soft iron 

 and stoel in reference to electric as in magnetic induc- 

 tion ; itnd U>tli, doubtless, are duo to the peculiar mole- 

 cular constitution of stool, arising from the presence of 

 carbon, and most probably, also, of nitrogen in it. 

 Knives, needles, and any other steel object having two 

 ends, may thus be at once rendered magnetic. 



Before describing electro-magnets of large size, and 

 elaetro-magnetio engines, which have been constructed 

 with a view to the production of motive power, we will 

 inquire into the circumstances which tend to increase or 

 diminish the effects which can thus bo produced by elec- 

 tri.-ity on iron or steel. 



Those may be epitomised as follows : 



1st The length of tho coil, and the thickness of the 

 wire composing it 



2nd. The number and size of tho plates of tho voltaic 

 lattery employed. 



3rd. Tho purity of the metals used in respect to the 

 copper wire and the iron core. 



1st. The Length, <Sx., of the Coil. It is a rule in 

 electrical science, that the longer the conductor, the 

 greater is the resistance afforded to tho passage of a 

 current; and it is equally true, that the greater the 

 transverse area of the conductor or, what is the same 

 thing, its thickness the less the resistance afforded by 

 the same length of wire. It has been found, that a copper 

 wire, of No. 16 gauge, has a conducting power of 100 ; 

 whilst a thinner one, of No. 18 gauge, has only a power 

 of 65. The following table, for which we are indebted 

 to an able work by T. Shaffner, Esq., of Kentucky, On 

 the Electric Telegraph, illustrates tho fact of tho dif- 

 ference of conducting power in wires of various thick- 

 nesses. Tho experiments were tried at tho Gutta- 

 Porcha Company s Works, in London, on wires each 

 100 miles long ; and these were submerged in tho 

 Regent's Canal every care having boon taken that the 

 external coat of gutta-percha was faultless. The figures 

 indicate the deflection of tho needle of a galvanometer* 

 for each gauge of wire employed. 



No. of nttcry Plate*. No. 18 Gauge. No. 10 Gauge. 



3 pairs 29 39 



6 50 0!t 



We thus observe how much tho resistance is increased 

 by a deficiency in the size of tho conducting wire ; and it 

 follows that a great length may bo, to some extent, com- 

 pensated for by an increased thickness of conducting 

 material. 



2nd. The Number and Size of Battery Plates. The 

 remarks we have just mado, and tho table quoted, lr.i.1 

 us to consider the effects produced by the size and 

 number of the voltaic batteries which may bo employed ; 

 and wo must hero point out that quantity of electricity 

 is produced by large plates, irrespective of the number ; 

 whilst intensity rosulta-from tho number of plates, rather 

 than their size. 



Now, the amount of magnetic effect produced on iron 

 by electricity, depends, theoretically, on the quantity of 

 the current alone ; but this is only true on the supposi- 

 tion that tho conductor offers not the slightest resistance 

 to tho passage of tho current. But, in practice, wo know 

 that such never can occur ; for tho longer tho wire, the 

 greater the resistance afforded ; and as each successive 

 coil increases tho amount of inductive effect, wo are 

 necessarily compelled to employ long wires in construct- 

 ing all electro-magnetic arrangements. To overcome tho 

 resistance of the wires as to their length, wo are there- 

 fore under the necessity of using, not only large plates, 

 but a number of them. Tho size of tho plate affords 

 tho necessary quantity of electricity ; and tho number of 

 tho plates assists that quantity, by their intensity, to 

 give out tho full force which one large plato would do 

 wore no resistance to be overcome. It therefore follows, 

 in practice, that both size and number of plates must 



See antf, p. 19j. 



be [considered in all attempts to produce the greatest 

 amount of electro-magnetic induction under most cir- 

 cumstances. Wo always presume that copper wire is 

 used in our experiments ; as, for various reasons, none 

 other, in most cases, is admissible. 



3rd. The Purity of the Metal used in both Coil and 

 Core. This is a point which has not received that at- 

 tention which it deserves. Indeed, the experiment* 

 necessary to afford accurate data, would require to be 

 of the most elaborate and extensive kind ; and tho 

 results, although of the greatest interest in a philo- 

 sophical point of view, could rarely have a pri 

 application for telegraphic and other purposes, owing to 

 the great expense and trouble which would have to be 

 incurred. Almost each length of copper wire which 

 may accidentally moot with, varies in this respect. In- 

 deed, we have found, that the longer the same wire has 

 been used as a conductor for powerful batteries, the 

 worse is its power of conduction. It undergoes a mole- 

 cular change ; becomes hard, and frequently breaks into 

 pieces with the slightest touch. Iron is subject to the 

 same influence ; and, like copper, it becomes coin]'! 

 "rotten" after having undergone lengthened el 

 action. Wo have frequently had occasion to remark 

 these facts whilst experimenting with apparatus in which 

 an intense current was employed ; and shall refer again to 

 the subject as wo proceed. 



HORSE-SHOE ELECTRO-MAGNETS, AND 

 ELECTRO-MAGNETIC MAC 1 1 1 N I X 



HAVING explained the principles on which electro-mag- 

 nets are constructed, and tho conditions which affect 

 their power, we proceed to make a practical application 

 of our remarks in reference to horse-shoe electro-mag- 

 nets and electro-magnetic machines, which have been 

 invented for tho purpose of obtaining a motive power 

 by employing voltaic electricity. It will not bo ne- 

 cessary for us to enter into any further theoretical 

 considerations : we shall therefore confine ourselvc.i 

 chiefly to the mechanical details of tho instruments to 

 be described. 



A horse-shoe electro-magnet has a similar advai, 

 to that we mentioned as possessed by tho permanent 

 steel horse-shoe magnet ;t for it lias its two poles in close 

 proximity, which, therefore, act in concert, and so pro- 

 duce a largo development of attractive power. The ends 

 should always be planed or ground as level as possible, 

 so that any object presented to them may come into as 

 close contact as possible. Wo shall give directions for 

 making them, and leave the question of sizo to tho 

 choice and means of our readers. The copper wire must 

 either bo covered with cotton, worsted, or silk ; tho first 

 material being the cheapest, and answering every pur- 

 pose. It is also necessary that the wire should alwa 

 as soft as possible, so that it may accommodate itself M 

 tho curves of the iron. Each turn should bo brought 

 tight up to that preceding it, so that there may be no 

 loose or "kinked" places throughout tho convolutions. 

 Tho iron part of a small electro-magnet may bo a ] 

 of rod iron, eighteen inches long, and an inch and a-half 

 in diameter. It is to bo bout so that its two ends may 

 bo brought within from two to three inches of each 

 other, in tlm manner represented in the following en- 

 graving. The winding-ou of tho wire is to bo com- 

 menced at the top of one end or pole, and to be con- 

 tinued for a depth of four or live inches towards tlm 

 bend. It may then be wound back towards tho top of 

 the pole again, and returned until the lower end is . 

 readied. One side will thus receive a coating <>f throe 

 coils of wire. This may then be passed over to the end 

 of tho rod nearest tho bend, care being taken to bind 

 the wire on it in tho same direction as on tho first arm ; 

 in fact, just as if tho wire hod been continuously wound 

 on a straight rod. Now, because tho centre poim 

 twoen the two extremities of a magnet does not exhibit ; 

 externally any magnetic effect, it is unnecessary to wind ' 

 any wire on it. Indeed, great advantage accrues by ' 

 '' condensing" tho wire on the poles of tho magnet, or, 

 t See ami, p. 211. 



