280
MAGNETS, BY LOGEMAX, &c. ; BROOKE’S PHOTOGRAPHIC APPARATUS. [Class X.
(England, No. 428, pp. 457, 458) have exhibited the mostpowerful steel magnets; it is satisfactory to learn thatboth these exhibitors are still applying themselves to theirfurther improvement.
Hearder (No. 439, p. 461) deserves special notice forthe attention he has devoted to ascertaining the receptiveand permanent magnetic powers of cast iroD, both inseparate plates and in their combinations.
As early as the year 1832, Dr. Scoresby commenced aseries of experiments on the magnetic properties of eastiron, but which, owing to the thickness of the massesemployed, did not yield decisive results; prior to the year1844, he repeated his experiments upon different kinds orcast iron, and found that “cast iron possesses considerablepowers of magnetism, both in capacity and retentiveness,though greatly inferior in both qualities to those of pro-perly-hardened steelwith several other interesting re-sults.* The general results arrived at by Dr. Scoresby,though calculated to show the unsuitable character of thespecimens and forms of the cast iron employed by him togeneral magnetic purposes, were yet of such an encou-raging nature, as to lead to the hope that more importantresults might be obtained by the employment of cast ironin large masses, and in different forms.
Mr. Hearder was led, in the course of his experiments,to consider that cast iron had not been subjected to a suffi-cient variety of tests, and he was still more confirmed inthis opinion, by observing how its mechanical propertiesvaried, according to the temperature of the different kindsof iron, and he concluded that the conditions which actedagainst its being used as a bar magnet might be madeavailable for one of the horse-shoe form. In 1843, hemade the magnet now in the Exhibition, which is stillpossessed of considerable power, as determined by theJury, f
Tiie following are the particulars of the magnets exhi-bited :—
Bookman (Netherlands, No. 87, p. 1147) has exhibitedthe most powerful permanent magnets in the Exhibition.
The following are the results of the experiments:—
One whose weight was 1 lb. -J oz. carried a weightof 10 lbs. 9£o z.
One whose weight was 6 lbs. J oz. carried a weightof 66 lbs. 3^oz.
One whose weight was 101 lbs. 12 oz. carried aweight of 436 lbs. 12 oz.
A Council Medal was awarded to Mr. Logeman.
The exhibitor of the next best magnets was IIexley(No. 428, pp. 457,458), and the results of the experimentsare as follows:—
A magnet weighing 2 lbs. 5 oz. carried a weight of31 lbs. f>£ oz.
A magnet weighing 8 lbs. 4 oz. carried a weight of56 lbs. 14 oz.
A magnet weighing 32 lbs. 6oz. carried a weight of120 lbs. 4joz.
Shaw and Sox, Sheffield (Class XXII., No. 216, p. 617),exhibited several magnets:—-
One whose weight was 7 oz. carried 4 lbs., and
One whose weight was 7 lbs. 2 oz. carried 61 lbs. 7| oz.
IIearder (No. 439, p. 461) has exhibited a cast-ironcompound horse-shoe permanent^ magnet. This magnetwas constructed with a view to its being applicable toevery purpose requiring high magnetic power. It con-sists of 24 plates, 2 inches wide and 0*19 inch thick,cast in the form of a horse-shoe, which is 16*75 inches inlength. The 24 plates are strongly fastened together;the poles, which are distant from each other 1*25 inches,are capped with soft iron, for the purpose of concentrating
* See Scoresbv’s “Magnctical Investigations,” Part IJ.,Chap, viii., pp. 330-347.
t It was described in the “Electrical Magazine,” October,1S45, vol. ii., p. 137.
X These magnets arc termed “ permanent,” in contra-distinction to the temporary electro-magnets.
the magnetic power; at the same time it renders thegrinding of the poles unnecessary.
The construction is simple. The bars being made ofiron, hardened as much as possible, require no preparationto adapt them for magnetization.
The economy of construction, as compared with a steelmagnet of the same dimensions, is estimated as 4 to 1.
It is designed to be made available for the constructionof electric machines for telegraphic or electro-chemicalpurposes, and was manufactured at Mr. Hearder’s esta-blishment at Plymouth.
Mr. Hearder observes that, whatever be the relativepowers of the plates previously to their being put together,as a mass they are found to undergo a considerable change;the two external magnets having their poles slightly re-versed, the two next being neutral, and the rest havingdirect polarity, being strongest in the centre, and gradu-ally diminishing towards the two external plates. Ontesting the individual powers of each magnet, after theirseveral combinations, for throe years, the sum of thewhole was less than 15 lbs., whilst collectively they werecapable of lifting a weight of more than 100 lbs.
Other particulars concerning this magnet are to befound in the Report of the Royal Cornwall PolytechnicInstitution for 1850.
This magnet was found to weigh 71 lbs. 8oz., and tocarry a weight of 120 lbs. 9 oz.
Mr. Hearder has exhibited a powerful horse-shoe per-manent steel magnet, intended for all purposes requiringhigh magnetic power.
It is composed of a large number of wide and extremelythin horse-shoe plates, cut out of thin sheet steel, tem-pered and hardened sufficiently to admit of their beingilattened with a hammer. The magnet weighs 39 lbs.,and Mr. Hearder says will support nearly 250 lbs., with around-faced keeper. It is intended for magueto-electricpurposes, where great power is required in a small space.This magnet was found to weigh 39ibs., and to carry aweight of 112 lbs. 9 oz.
Rundell (No. 438a, p. 461) exhibits a carbonized cast-iron magnet; the carbonization having been performedby prussiate of potash and oil. This magnet is offered asan improvement on Mr. Ilearder's cast-iron magnet, asshown at the last Polytechnic Exhibition. On trial, thismagnet was found to weigh 27 lbs. 8oz., and to carry35 lbs. 9oz.
Magnelical Instruments.
Brooke (No. 144, pp. 422—426) exhibits photographicapparatus for the self-registration of the changes of posi-tion of the declination magnet, of the horizontal forcemagnet, and of the vertical force magnet.
The three magnets are so placed, that the residual effectof eacli pair upon the third is a minimum, which is to bedetermined by experiment.
The principle is the same for all, viz., wrapping pre-pared photographic paper around a cylinder whose axis isplaced parallel to the direction of movement to be regis-tered, and which is turned round uniformly by clockwork.
The light, from a gas-lamp, passes through a smallaperture placed near it, and falls upon a concave mirrorof speculum metal, which rests in a stirrup firmly con-nected to the magnet, so that it partakes of all the angularmovements of the magnet. The pencil of light is thendeflected from the mirror to a plano-convex lens, placedparallel to the axis of the cylinder, and near to it: thislens condenses the line of light to a definite spot of lighton the paper. This spot of light, therefore, moves withtlie movements to be registered,—to the right and left, ina horizontal plane, in the case of the declination magnetand horizontal force magnet; and up and down, in a ver-tical plane, to register the movement of the vertical forcemagnet; the cylinder, as before stated, being turned roundbj* clockwork. Consequently, there is traced upon thepaper a curve, of which the abscissa, measured in thedirection of a line round the cylinder, is proportional tothe time, while the ordinate, measured in the direction ofthe axis of the cylinder, is proportional to the movementof the magnet.
A base line, or a line from which to measure the ordi-