« EelmineJätka »
thus in fig. 5 we see that the curves for steel are much more steep than that of iron, and would thus give greater values to
r in the formula, a result to be expected. We also observe in both figures the great change in distribution due to the direction of magnetization. In the case of the electro-magnet this amounts to little more than a change in scale; but in the permanent magnet there is a real change of form in the curve. It seems probable that this change
of form would be done away é
with by using a sufficient mag.
netizing-power or magnetizing Results from electro-magnets :- by application of permanent A. Iron, from Table XI. B. Steel, from Table XII, magnetized magnets; for it is probable that same as originally.
the fall in the curve E is due C. Steel, from Table XII, magnetized to the magnetizing-force hav.
opposite to its original magnetism. ing been sufficient to change the polarity completely at the center, but only partially at the ends.
On comparing the distribution on electro-magnets with that on permanent magnets, we perceive that the curve is steeper
toward the end in electro-magnets than in permanent magnets. At first I thought it might be due to the direct action of the helix, but on trial found that the latter was almost inappreciable. I do not at present know the explanation
of it. Results from steel permanent magnets:
As before mentioned, CouD. Magnetized in its original direction,
lomb has made many experiTable XII.
ments on the distribution of E. Magnetized opposite to its original magnetism on permanent mag.
direction, Table XII.
nets, and so I shall only con.
sider this subject briefly. I have already given one or two results in Table XII.
The following tables were taken from two exactly similar Stub's steel rods not hardened, one of which was subsequently used in the experiments of Table XII. They were 12.8 inches long and .19 inch in diameter.
The coincidence of these observations with the formula is very remarkable, but still we see a little tendency in the end observation to rise above the value given by the formula.
412=:117(10.2 03(1–– 10.2 035). In equation (7), and also from Green’s formula, we have seen that for a given quality and temper of steel is a constant. From Coulomb's experiments on a steel bar .176 inch in diameter whose quality and temper is unknown, though it was
probably hardened, Green has calculated the value of this constant and obtained •05482, which was found from the French inch as the unit of length, but which is constant for all systems. From Tables XIII. and XIV. we find the value of r to be ·4674,
rd w whence =.04440 for steel not hardened. As the steel becomes harder, this quantity increases and can probably reach about twice this for very hard steel.
Hard steel, B.
To show the effect of hardening, I broke the bar used in Table XIV. at the center, thus producing two bars 6:4 inches
ilts can the changeted to give he curves there to show have attem Lave me
loog. One of these halves was hardened till it could scarcely be scratched by a file, but the other half was left unaltered. The following table gives the distribution, using the same unit as that of Tables XIII. and XIV. The bars were so short that the results can hardly be relied on; but they will at least suffice to show the change.
In fig. 7 I have attempted to give the curve of distribution from Table XV, and have made the curves coincide with
observation as nearly as possible, making a small allowance, however, for the errors introduced by the shortness of the bar. It is seen that the effect of hardening in a bar of these dimensions is to increase the quantity of magnetism, but especially that near the end. Had the bar been very long, no increase in the total quantity of magnetism would have taken place, but the distribution would have been changed. Hence from this we deduce the important fact that hardening is most useful for short magnets. And it would seem that almost the only use in
hardening magnets at all is to Results from permanent magnets.
concentrate the magnetism and A. Soft steel.
to reduce the weight. And inB. Hard steel.
deed I have made magnets from iron wire whose magnetization at the central section was just as intense as in a steel wire of the same size ; but to all appearances it was less strongly magnetized than the steel be. cause the magnetism was more diffused ; and as the magnetism was not distributed so nearly at the end as in the steel, its magnetic moment and time of vibration was less.
It is for these reasons that many makers of surveyors' compasses find it unnecessary to harden the needles, seeing that these are long and thin.
We might deduce all these facts from the formulæ on the assumption that r is greater, the harder the iron or steel.
Having thus considered briefly the distribution on electromagnets and steel magnets, and found that the formulæ represent it in a general way, we may now use them for solving a few questions that we desire to know, though only in an approximate manner.
[To be continued.]
ART. III. — On Recent Researches in Sound; by WM. B. TAYLOR.
THAT two so eminent physicists as Professor Tyndall in England, and Professor Henry in our own country, should have been for some time past (and almost simultaneously) engaged in investigating the aberrant actions of Sound, with especial reference to securing increased efficiency to the national systems of Fog-signaling, is a noteworthy circumstance, and one of no slight practical importance. In view of the many disastrous marine accidents resulting from fogs on either coast, every thoughtful mind must regard with profound interest a series of researches requiring so much patient labor for the attainment of new and accurate information on the subject, and so high a degree of scientific sagacity and skill for its right interpretation.
As somewhat different explanations bave been offered by these two distinguished observers to account for certain abnor: mal phenomena of sound, a concise statement of the facts and views respectively announced, will interest the general reader. The records of these investigations are, on the one side, the Philosophical Transactions of the Royal Society of London for the year 1874, vol. clxiv, page 183, “ On the Atmosphere as a Vehicle of Sound," by John Tyndall, LL.D., F.R.S., a com: munication read February 12, 1874; and on the other side, the Annual Report of the Light House Board of the United States for the year 1874; the Appendix to which is an account of the operations of the Board relative to Fog-Signals, by Joseph Henry, Chairman of the Light House Board. In addition to these principal sources of information, reference will be made to an interesting communication read before the Royal Society, April 23, 1874, “ On the Refraction of Sound," by Professor Osborne Reynolds, and published in the Proceedings of the Royal Society for 1874. The salient points of the observations are selected, and are here arbitrarily designated by bracketed numbers, to facilitate comparisons.
Ten years ago, or in 1865, Professor Henry commenced his investigations on the subject of Sound in connection with fog. signals, at the Light House station near New Haven, Connecticut. Omitting here his careful experiments in regard to the character of the various instruments employed, the principal results then obtained, were the following:
[1.] The reflection of sound was observed to be very imperfect and inexact. A large concave reflector with a smoothly
could 72on." at distance" projecte of pa
plastered surface of 64 square feet, produced a sensible increase of effect in the sound, within a distance of 500 yards in front of the signal: beyond this distance, the difference became imperceptible. It appeared that “while feeble sounds at small distances are reflected as rays of light are, waves of powerful sound spread laterally, and even when projected from the mouth of a trumpet, at a great distance tend to embrace the wbole circle of the horizon." (L. H. Rep., p. 88.) A trumpet, however, which could be heard six iniles in front (in the direction of the axis) was heard only three miles in the rear. (p. 92.)
[2.] “ For determining the relative power of the instruments, the use of two vessels had been obtained.” The instruments at the light-house station were a large bell, a steam-whistle 6 inches in diameter, a double whistle, “improperly called a steam gong," 12 inches in dianieter, the cups being 20 and 14 inches deep, producing the harmonic interval of a tone and its fifth, and a Daboll trumpet operated by a hot air engine. The blow-off sound from the "exhaust” of the air engine was also noted. “The penetrating power of the trumpet was nearly double that of the whistle." (Rep., p. 90.) The order of audible range on the first day was found to be 1st, trumpet, 2nd, exhaust, 3rd, bell, the whistle not being sounded. On the sec. ond day, 1st, trumpet and “gong," 2nd, whistle, 3rd, exhaust. In the rear the trumpet was heard no farther than the whistle. On the third day, the order was similar, -1st, truinpet, 2nd, whistle, 3rd, exhaust, 4th, bell. (p. 91.) The opportunity was unfavorable to the observation of these sounds when they were moving directly with the wind.
(3.) Simultaneous observations from two vessels sailing in nearly opposite directions, showed that the sound did not extend against the wind so far as in the direction of the wind; and on subsequent days, results obtained from sounds moving nearly against the wind, and at right-angles to it, indicated that an opposing wind, when light, obstructed sound less than when stronger, and that wind at right-angles to the sound, permitted it to be heard farther. (Rep., p. 92.)
[4.] “During this series of investigations an interesting fact was discovered, namely, a sound moving against the wind, inaudible to the ear on the deck of the schooner, was heard by ascending to the mast-head.” (p. 92.) These results were obtained in 1865.
[5.] An experiment subsequently made at Washington during a fog, with a small clock-work alarm bell, indicated that the fog did not absorb sound; though want of the opportunity of a comparative observation prevented the result from being entirely satisfactory. (p. 93.)
In 1867, the principal object of investigation was a compari