ELECTRO-MAGNETS.] UNDULATORY FORCES. E LECTRO-MAGNETISM. 



251 



rather, near to the outer extremities of the iron, by 

 winding extra coils on those parts. It is there that the 

 greatest inductive power is manifested, and where, there- 

 fore, the greater effect should be produced. It may 

 here be remarked, that electro-magnets, like ordinary 

 magnets, have their poles at a distance from their extre- 

 mities ; hence the coating of covered wire is never carried 

 to the extreme end of each arm. 



The following engraving illustrates the appearance of 

 an electro-magnet when complete. Right and left are 

 its poles, N and S. Beneath them is the keeper, to 



Fig. 12 



which a hook is attached, and to which heavy weights 

 may be hung. From N and S proceed the two wires, 

 which are to be attached to the wires of a voltaic 

 battery when the electro-magnet is in use; and by 

 means of which the current passes to the coils, round 

 the iron. When very large magnets are made, it is 

 usual to divide the coil into separate portions, and to 

 collect the various ends of the coils together, so as to 

 form one terminal for each end of all the coils. By 

 these means the current of electricity circulates as maiiy 

 times round the iron as there are convolutions of wire 

 placed round it, but has not to travel in cue continuous 

 length for the whole distance. By this arran 

 the resistance of great lengths of wire is done away with, 

 and yet the full effect of the quantity of the battery 

 power is advantageously employed.* The end wires 

 of the electro-magnet should never bo twisted about, as 

 they soon get very brittle ; and if the wire be once 

 broken close to the iron, and inside the coil, the magnet 

 becomes useless, and must be recoiled by the wire. It is 

 a good plan to fold some coarse brown paper, or a tliin 

 sheet of gutta-percha, on the iron arms before coiling 

 on the covered copper wire, as this prevents contact 

 between the metals, in case the covering is in any part 

 defective. The coils may be bound on wooden cases, and 

 these can then be slipped on to the iron arms of the 

 electro-magnet, and removed when done with. As, 

 however, the thickness of the wood removes the covered 

 wire from the iron, and so increases the distance of the 

 inductive power from its surface, a loss of power neces- 

 sarily results from this arrangement. The attractive, 

 or holding power of an electro-magnet, depends on 

 its size, and on the strength of the battery employed. 

 Such an arrangement as we have described, although 

 small, will sustain at least one hundredweight, when a 

 current from twenty cells of Smee's, or four to eight cells 

 of (Jrove's battery, is sent through the wires. 



The largest electro-magnet which wo have yet seen, 

 and perhaps the most powerful which has been made, 

 was one which wo purchased from the Panopticon, at the 

 public sale of effects, for the Polytechnic Institution in 

 Glasgow. The wire and iron had been made of the 

 purest metal which could be procured, and both were 

 well annealed. The wire was a quarter of an inch thick ; 

 and the total quantity employed weighed nearly two hun- 

 Scc ante, p. 250. 



dred weight. The iron, which was of the usual horse- 

 shoe form, weighed about half a ton ; and each arm was 

 three feet high, and six inches in diameter : the remain- 

 ing length of the iron rod formed the bend. We have 

 repeatedly employed fifty and 100 cells of Grove's battery 

 as the source of the voltaic current ; and, under such 

 circumstances, we have attempted in vain to remove the 

 keeper from the poles by opposing a weight of over three 

 tons. When two hundredweight of iron nails, each 

 of which measured six inches long, and of the usual 

 substance, were cast on the pole, every nail was so power- 

 fully attracted, as that they could scarcely be pulled 

 apart, and could not be pushed away from each other. 



But there is a most remarkable fact in reference to all 

 magnets ; which is, that the attractive power decreases in 

 a most rapid ratio. In the case of the large magnet wo 

 have just described, the attractive force, at three inches 

 from either pole, was very trifling not exceeding a few 

 ounces. Thus we observe that the decrease in three 

 inches distance from its source, was, beyond all com- 

 parison, greater in ratio than that of any other force with 

 which we are acquainted ; and this, unfortunately, has to 

 a large extent prevented this agent from being employed 

 as a motive power. In all kinds of magnets, the at- 

 tractive power is greatest in those which have the longest 

 arms; and this we should naturally expect, from the 

 fact that the force diminishes to nothing as we approach 

 to a point equi-distant between the poles. It is there- 

 fore better, in making an electro-magnet, to employ the 

 wire so that it may cover the poles, rather than tho 

 whole length of tho arms; and tho neglect of this in 

 the construction of the large magnet of which wo have 

 hern s] Baking, doubtless prevented its maximum of 

 power being obtained. 



ELECTRO-MAGNETIC ENGINES. Wo shall now give a 

 general outline of some of the attempts which have been 

 made to obtain a power for driving machinery, <tc., by 

 means of electro-magnets : but before doing BO, it is to 

 bo remarked, that the best-constructed arrangements 

 have liithorto been complete failures in an engineering 

 and economic point of view. This has arisen from three 

 causes. 1st, the extremely small distance at which tho 

 attraction acts, and its rapid ratio of decrease, which wo 

 have already discussed ; 2nd, tho interference of resi- 

 duary magnetism, f which prevents an instantaneous 

 change in the direction of tho attractive power ; and, 

 3rd, the expense of voltaic batteries not in their first 

 cost, but in maintaining their power. In days such as 

 our own, when inductive and experimental science, to- 

 gether with mechanical skill, are found ill so high a 

 degree of perfection, it seems most extraordinary that 

 no success has as yet attended the numerous efforts 

 which have been made to procure it. 



When we compare the progress which has been made 

 in steam machinery, with that which has attended that 

 of the electro-magnetic arrangements, wo can find no 

 approximation in the hitter. The mechanical difficulty 

 results chiefly from the fact, that the rapid diminution 

 of the attractive power of an electro-magnet, does not 

 permit of any of the ordinary appliances for converting 

 force into motion to be used. For instance, the stroke 

 of the piston of the steam-engine may bo of any length 

 which the engineer chooses to employ. All ho need do 

 is to lengthen the cylinder, or supply the steam each of 

 which can be effected to any extent which may be de- 

 sired ; and the piston can be driven to and fro, acting 

 on a crank of half its length. Now, in electro-magnetic 

 engines, *he greatest direct stroke which can be em- 

 ployed between two electro-magnets, cannot exceed 

 (without the intervention of levers) a quarter to half an 

 inch ; and, of course, a crank of half that length, and 

 of the 7iecessary strength, is a physical impossibility in 

 an engineering point of view. 



It does not matter in what form the motion Is ob- 

 tained by any of the inventions which have been made, 

 for still the rapid ratio of tho decrease of force is con- 

 stantly active. We fear, that unless some means may be 

 discovered by which this power can bo extended in a 

 t See ante, p. 249. 



