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diff --git a/old/5192-pdf.pdf b/old/5192-pdf.pdf Binary files differnew file mode 100644 index 0000000..bbbe9a3 --- /dev/null +++ b/old/5192-pdf.pdf diff --git a/old/5192-t.tex/5192-t.tex b/old/5192-t.tex/5192-t.tex new file mode 100644 index 0000000..fb05800 --- /dev/null +++ b/old/5192-t.tex/5192-t.tex @@ -0,0 +1,764 @@ +\documentclass[12pt]{article} +\usepackage{graphicx} +%\usepackage{amssymb} + +\textwidth = 6.5 in +\textheight = 9 in +\oddsidemargin = 0.0 in +\evensidemargin = 0.0 in +\topmargin = 0.0 in +\headheight = 0.0 in +\headsep = 0.0 in + +%\newtheorem{theorem}{Theorem} +%\newtheorem{corollary}[theorem]{Corollary} +\newtheorem{definition}{Definition} + +\renewcommand{\thefootnote}{\fnsymbol{footnote}} + +\title{On a Dynamical Top, for exhibiting the phenomena of the motion +of a system of invariable form about a fixed point, with some +suggestions as to the Earth's motion} +\author{James Clerk Maxwell} +\date{\small [From the \emph{Transactions of the Royal Society of Edinburgh}, +Vol. XXI. Part IV.] \\ +(Read 20th April, 1857.)} + +\begin{document} +\maketitle + +To those who study the progress of exact science, the common +spinning-top is a symbol of the labours and the perplexities of men +who had successfully threaded the mazes of the planetary motions. The +mathematicians of the last age, searching through nature for problems +worthy of their analysis, found in this toy of their youth, ample +occupation for their highest mathematical powers. + +No illustration of astronomical precession can be devised more perfect +than that presented by a properly balanced top, but yet the motion of +rotation has intricacies far exceeding those of the theory of +precession. + +Accordingly, we find Euler and D'Alembert devoting their talent and +their patience to the establishment of the laws of the rotation of +solid bodies. Lagrange has incorporated his own analysis of the +problem with his general treatment of mechanics, and since his time +M.~Poins\^ot has brought the subject under the power of a more +searching analysis than that of the calculus, in which ideas take the +place of symbols, and intelligible propositions supersede equations. + +In the practical department of the subject, we must notice the +rotatory machine of Bohnenberger, and the nautical top of Troughton. +In the first of these instruments we have the model of the Gyroscope, +by which Foucault has been able to render visible the effects of the +earth's rotation. The beautiful experiments by which Mr J. Elliot has +made the ideas of precession so familiar to us are performed with a +top, similar in some respects to Troughton's, though not borrowed from +his. + +The top which I have the honour to spin before the Society, differs +from that of Mr Elliot in having more adjustments, and in being +designed to exhibit far more complicated phenomena. + +The arrangement of these adjustments, so as to produce the desired +effects, depends on the mathematical theory of rotation. The method +of exhibiting the motion of the axis of rotation, by means of a +coloured disc, is essential to the success of these adjustments. This +optical contrivance for rendering visible the nature of the rapid +motion of the top, and the practical methods of applying the theory +of rotation to such an instrument as the one before us, are the +grounds on which I bring my instrument and experiments before the +Society as my own. + +I propose, therefore, in the first place, to give a brief outline of +such parts of the theory of rotation as are necessary for the +explanation of the phenomena of the top. + +I shall then describe the instrument with its adjustments, and the +effect of each, the mode of observing of the coloured disc when the +top is in motion, and the use of the top in illustrating the +mathematical theory, with the method of making the different +experiments. + +Lastly, I shall attempt to explain the nature of a possible variation +in the earth's axis due to its figure. This variation, if it exists, +must cause a periodic inequality in the latitude of every place on the +earth's surface, going through its period in about eleven months. The +amount of variation must be very small, but its character gives it +importance, and the necessary observations are already made, and only +require reduction. + +\section*{On the Theory of Rotation.} + +The theory of the rotation of a rigid system is strictly deduced from +the elementary laws of motion, but the complexity of the motion of the +particles of a body freely rotating renders the subject so intricate, +that it has never been thoroughly understood by any but the most +expert mathematicians. Many who have mastered the lunar theory have +co~ne to erroneous conclusions on this subject; and even Newton has +chosen to deduce the disturbance of the earth's axis from his theory +of the motion of the nodes of a free orbit, rather than attack the +problem of the rotation of a solid body. + +The method by which M. Poins\^ot has rendered the theory more +manageable, is by the liberal introduction of ``appropriate ideas,'' +chiefly of a geometrical character, most of which had been rendered +familiar to mathematicians by the writings of Monge, but which then +first became illustrations of this branch of dynamics. If any further +progress is to be made in simplifying and arranging the theory, it +must be by the method which Poins\^ot has repeatedly pointed out as +the only one which can lead to a true knowledge of the subject,---that +of proceeding from one distinct idea to another instead of trusting to +symbols and equations. + +An important contribution to our stock of appropriate ideas and +methods has lately been made by Mr R. B. Hayward, in a paper, ``On a +Direct Method of estimating Velocities, Accelerations, and all similar +quantities, with respect to axes, moveable in any manner in Space.'' +(\emph{Trans. Cambridge Phil. Soc} Vol. x. Part I.) + +\footnote[1]{7th May 1857. The paragraphs marked thus have been +rewritten since the paper was read.}In this communication I intend to +confine myself to that part of the subject which the top is intended +io illustrate, namely, the alteration of the position of the axis in a +body rotating freely about its centre of gravity. I shall, therefore, +deduce the theory as briefly as possible, from two considerations +only,---the permanence of the original \emph{angular momentum} in +direction and magnitude, and the permanence of the original \emph{vis +viva}. + +\footnotemark[1]The mathematical difficulties of the theory of +rotation arise chiefly from the want of geometrical illustrations and +sensible images, by which we might fix the results of analysis in our +minds. + +It is easy to understand the motion of a body revolving about a fixed +axle. Every point in the body describes a circle about the axis, and +returns to its original position after each complete revolution. But +if the axle itself be in motion, the paths of the different points of +the body will no longer be circular or re-entrant. Even the velocity +of rotation about the axis requires a careful definition, and the +proposition that, in all motion about a fixed point, there is always +one line of particles forming an instantaneous axis, is usually given +in the form of a very repulsive mass of calculation. Most of these +difficulties may be got rid of by devoting a little attention to the +mechanics and geometry of the problem before entering on the +discussion of the equations. + +Mr Hayward, in his paper already referred to, has made great use of +the mechanical conception of Angular Momentum. + +\begin{definition} +The Angular Momentum of a particle about an axis is measured by the +product of the mass of the particle, its velocity resolved in the +normal plane, and the perpendicular from the axis on the direction of +motion. +\end{definition} + +\footnotemark[1]The angular momentum of any system about an axis is +the algebraical sum of the angular momenta of its parts. + +As the \emph{rate of change} of the \emph{linear momentum} of a +particle measures the \emph{moving force} which acts on it, so the +\emph{rate of change} of \emph{angular momentum} measures the +\emph{moment} of that force about an axis. + +All actions between the parts of a system, being pairs of equal and +opposite forces, produce equal and opposite changes in the angular +momentum of those parts. Hence the whole angular momentum of the +system is not affected by these actions and re-actions. + +\footnotemark[1]When a system of invariable form revolves about an +axis, the angular velocity of every part is the same, and the angular +momentum about the axis is the product of the \emph{angular velocity} +and the \emph{moment of inertia} about that axis. + +\footnotemark[1]It is only in particular cases, however, that the +\emph{whole} angular momentum can be estimated in this way. In +general, the axis of angular momentum differs from the axis of +rotation, so that there will be a residual angular momentum about an +axis perpendicular to that of rotation, unless that axis has one of +three positions, called the principal axes of the body. + +By referring everything to these three axes, the theory is greatly +simplified. The moment of inertia about one of these axes is greater +than that about any other axis through the same point, and that about +one of the others is a minimum. These two are at right angles, and +the third axis is perpendicular to their plane, and is called the mean +axis. + +\footnotemark[1]Let $A$, $B$, $C$ be the moments of inertia about the +principal axes through the centre of gravity, taken in order of +magnitude, and let $\omega_1$ $\omega_2$ $\omega_3$ be the angular +velocities about them, then the angular momenta will be $A\omega_1$, +$B\omega_2$, and $C\omega_3$. + +Angular momenta may be compounded like forces or velocities, by the +law of the ``parallelogram,'' and since these three are at right angles +to each other, their resultant is +% +\begin{equation} +\sqrt{A^2\omega_1^2 + B^2\omega_2^2 + C^2\omega_3^2} = H +\end{equation} +% +and this must be constant, both in magnitude and direction in space, +since no external forces act on the body. + +We shall call this axis of angular momentum the \emph{invariable +axis}. It is perpendicular to what has been called the invariable +plane. Poins\^ot calls it the axis of the couple of impulsion. The +\emph{direction-cosines} of this axis in the body are, + +\begin{displaymath} +\begin{array}{c c c} +\displaystyle l = \frac{A\omega_1}{H}, & +\displaystyle m = \frac{B\omega_2}{H}, & +\displaystyle n = \frac{C\omega_3}{H}. +\end{array} +\end{displaymath} + +Since $I$, $m$ and $n$ vary during the motion, we need some additional +condition to determine the relation between them. We find this in the +property of the \emph{vis viva} of a system of invariable form in +which there is no friction. The \emph{vis viva} of such a system must +be constant. We express this in the equation + +\begin{equation} +A\omega_1^2 + B\omega_2^2 + C\omega_3^2 = V +\end{equation} + +Substituting the values of $\omega_1$, $\omega_2$, $\omega_3$ in terms +of $l$, $m$, $n$, + +\begin{displaymath} +\frac{l^2}{A} + \frac{m^2}{B} + \frac{n^2}{C} = \frac{V}{H^2}. +\end{displaymath} + +Let $1/A = a^2$, $1/B = b^2$, $1/c = c^2$, $V/H^2 = e^2$, and this +equation becomes +% +\begin{equation} +a^2l^2 + b^2m^2 + c^2n^2 = e^2 +\end{equation} +% +and the equation to the cone, described by the invariable axis within +the body, is + +\begin{equation} +(a^2 - e^2) x^2 + (b^2 - e^2) y^2 + (c^2 - e^2) z^2 = 0 +\end{equation} + +The intersections of this cone with planes perpendicular to the +principal axes are found by putting $x$, $y$, or $z$, constant in this +equation. By giving $e$ various values, all the different paths of +the pole of the invariable axis, corresponding to different initial +circumstances, may be traced. + +\begin{figure} +\includegraphics[width=\textwidth]{fig1.png} +\caption{} +\end{figure} + +\footnotemark[1]In the figures, I have supposed $a^2 = 100$, $b^2= +107$, and $c^2= 110$. The first figure represents a section of the +various cones by a plane perpendicular to the axis of $x$, which is +that of greatest moment of inertia. These sections are ellipses +having their major axis parallel to the axis of $b$. The value of +$e^2$ corresponding to each of these curves is indicated by figures +beside the curve. The ellipticity increases with the size of the +ellipse, so that the section corresponding to $e^2 = 107$ would be two +parallel straight lines (beyond the bounds of the figure), after which +the sections would be hyperbolas. + +\begin{figure} +\includegraphics[width=\textwidth]{fig2.png} +\caption{} +\end{figure} + +\footnotemark[1]The second figure represents the sections made by a +plane, perpendicular to the \emph{mean} axis. They are all +hyperbolas, except when $e^2= 107$, when the section is two +intersecting straight lines. + +\begin{figure} +\includegraphics[width=\textwidth]{fig3.png} +\caption{} +\end{figure} + +The third figure shows the sections perpendicular to the axis of least +moment of inertia. From $e^2 = 110$ to $e^2 = 107$ the sections are +ellipses, $e^2 = 107$ gives two parallel straight lines, and beyond +these the curves are hyperbolas. + +\begin{figure} +\includegraphics[width=\textwidth]{fig4.png} +\caption{} +\end{figure} + +\footnotemark[1]The fourth and fifth figures show the sections of the +series of cones made by a cube and a sphere respectively. The use of +these figures is to exhibit the connexion between the different curves +described about the three principal axes by the invariable axis during +the motion of the body. + +\begin{figure} +\includegraphics[width=\textwidth]{fig5.png} +\caption{} +\end{figure} + +\footnotemark[1]We have next to compare the velocity of the invariable +axis with respect to the body, with that of the body itself round one +of the principal axes. Since the invariable axis is fixed in space, +its motion relative to the body must be equal and opposite to that of +the portion of the body through which it passes. Now the angular +velocity of a portion of the body whose direction-cosines are $l$, +$m$, $n$, about the axis of $x$ is + +\begin{displaymath} +\frac{\omega_1}{1 - l^2} - + \frac{l}{1 - l^2}(l\omega_1 + m\omega_2 + n\omega-3). +\end{displaymath} + +Substituting the values of $\omega_1$, $\omega_2$, $\omega_3$, in +terms of $l$, $m$, $n$, and taking account of equation (3), this +expression becomes + +\begin{displaymath} +H\frac{(a^2 - e^2)}{1 - l^2}l. +\end{displaymath} + +Changing the sign and putting $\displaystyle l = +\frac{\omega_1}{a^2H}$ we have the angular velocity of the invariable +axis about that of $x$ +% +\begin{displaymath} += \frac{\omega_1}{1 - l^2} \frac{e^2 - a^2}{a^2}, +\end{displaymath} +% +always positive about the axis of greatest moment, negative about that +of least moment, and positive or negative about the mean axis +according to the value of $e^2$. The direction of the motion in every +case is represented by the arrows in the figures. The arrows on the +outside of each figure indicate the direction of rotation of the body. + +\footnotemark[1]If we attend to the curve described by the pole of the +invariable axis on the sphere in fig. 5, we shall see that the areas +described by that point, if projected on the plane of $yz$, are swept +out at the rate + +\begin{displaymath} +\omega_1 \frac{e^2 - a^2}{a^2}. +\end{displaymath} + +Now the semi-axes of the projection of the spherical ellipse described +by the pole are + +\begin{displaymath} +\sqrt{\frac{e^2 - a^2}{b^2 - a^2}} + \hspace{1cm}\textrm{and}\hspace{1cm} +\sqrt{\frac{e^2 - a^2}{c^2 - a^2}}. +\end{displaymath} + +Dividing the area of this ellipse by the area described during one +revolution of the body, we find the number of revolutions of the body +during the description of the ellipse--- + +\begin{displaymath} += \frac{a^2}{\sqrt{b^2 - a^2}\sqrt{c^2 - a^2}}. +\end{displaymath} + +The projections of the spherical ellipses upon the plane of $yz$ are +all similar ellipses, and described in the same number of revolutions; +and in each ellipse so projected, the area described in any time is +proportional to the number of revolutions of the body about the axis +of $x$, so that if we measure time by revolutions of the body, the +motion of the projection of the pole of the invariable axis is +identical with that of a body acted on by an attractive central force +varying directly as the distance. In the case of the hyperbolas in +the plane of the greatest and least axis, this force must be supposed +repulsive. The dots in the figures 1, 2, 3, are intended to indicate +roughly the progress made by the invariable axis during each +revolution of the body about the axis of $x$, $y$ and $z$ +respectively. It must be remembered that the rotation about these +axes varies with their inclination to the invariable axis, so that the +angular velocity diminishes as the inclination increases, and +therefore the areas in the ellipses above mentioned are not described +with uniform velocity in absolute time, but are less rapidly swept out +at the extremities of the major axis than at those of the minor. + +\footnotemark[1]When two of the axes have equal moments of inertia, or +$b = c$, then the angular velocity $\omega_1$ is constant, and the +path of the invariable axis is circular, the number of revolutions of +the body during one circuit of the invariable axis, being + +\begin{displaymath} +\frac{a^2}{b^2 - a^2} +\end{displaymath} + +The motion is in the same direction as that of the rotation, or in the +opposite direction, according as the axis of $x$ is that of greatest +or of least moment of inertia. + +\footnotemark[1]Both in this case, and in that in which the three axes +are unequal, the motion of the invariable axis in the body may be +rendered very slow by diminishing the difference of the moments of +inertia. The angular velocity of the axis of $x$ about the invariable +axis in space is +% +\begin{displaymath} +\omega_1\frac{e^2 - a^2l^2}{a^2(1 - l^2)}, +\end{displaymath} +% +which is greater or less than $\omega_1$, as $e^2$ is greater or less +than $a^2$, and, when these quantities are nearly equal, is very +nearly the same as $\omega_1$ itself. This quantity indicates the +rate of revolution of the axle of the top about its mean position, and +is very easily observed. + +\footnotemark[1]The \emph{instantaneous axis} is not so easily +observed. It revolves round the invariable axis in the same time with +the axis of $x$, at a distance which is very small in the case when +$a$, $b$, $c$, are nearly equal. From its rapid angular motion in +space, and its near coincidence with the invariable axis, there is no +advantage in studying its motion in the top. + +\footnotemark[1]By making the moments of inertia very unequal, and in +definite proportion to each other, and by drawing a few strong lines +as diameters of the disc, the combination of motions will produce an +appearance of epicycloids, which are the result of the continued +intersection of the successive positions of these lines, and the cusps +of the epicycloids lie in the curve in which the instantaneous axis +travels. Some of the figures produced in this way are very pleasing. + +In order to illustrate the theory of rotation experimentally, we must +have a body balanced on its centre of gravity, and capable of having +its principal axes and moments of inertia altered in form and position +within certain limits. We must be able to make the axle of the +instrument the greatest, least, or mean principal axis, or to make it +not a principal axis at all, and we must be able to \emph{see} the +position of the invariable axis of rotation at any time. There must +be three adjustments to regulate the position of the centre of +gravity, three for the magnitudes of the moments of inertia, and three +for the directions of the principal axes, nine independent +adjustments, which may be distributed as we please among the screws of +the instrument. + +\begin{figure} +\begin{center} +\includegraphics[width=0.8\textwidth]{fig6.png} +\end{center} +\caption{} +\end{figure} + +The form of the body of the instrument which I have found most +suitable is that of a bell (fig. 6). $C$ is a hollow cone of brass, +$R$ is a heavy ring cast in the same piece. Six screws, with heavy +heads, $x$, $y$, $z$, $x'$, $y'$, $z'$, work horizontally in the ring, +and three similar screws, $l$, $m$, $n$, work vertically through the +ring at equal intervals. $AS$ is the axle of the instrument, $SS$ is +a brass screw working in the upper part of the cone $C$, and capable +of being firmly clamped by means of the nut $c$. $B$ is a cylindrical +brass bob, which may be screwed up or down the axis, and fixed in its +place by the nut $b$. + +The lower extremity of the axle is a fine steel point, finished +without emery, and afterwards hardened. It runs in a little agate cup +set in the top of the pillar $P$. If any emery had been embedded in +the steel, the cup would soon be worn out. The upper end of the axle +has also a steel point by which it may be kept steady while spinning. + +When the instrument is in use, a coloured disc is attached to the +upper end of the axle. + +It will be seen that there are eleven adjustments, nine screws in the +brass ring, the axle screwing in the cone, and the bob screwing on the +axle. The advantage of the last two adjustments is, that by them +large alterations can be made, which are not possible by means of the +small screws. + +The first thing to be done with the instrument is, to make the steel +point at the end of the axle coincide with the centre of gravity of +the whole. This is done roughly by screwing the axle to the right +place nearly, and then balancing the instrument on its point, and +screwing the bob and the horizontal screws till the instrument will +remain balanced in any position in which it is placed. + +When this adjustment is carefully made, the rotation of the top has no +tendency to shake the steel point in the agate cup, however irregular +the motion may appear to be. + +The next thing to be done, is to make one of the principal axes of the +central ellipsoid coincide with the axle of the top. + +To effect this, we must begin by spinning the top gently about its +axle, steadying the upper part with the finger at first. If the axle +is already a principal axis the top will continue to revolve about its +axle when the finger is removed. If it is not, we observe that the +top begins to spin about some other axis, and the axle moves away from +the centre of motion and then back to it again, and so on, alternately +widening its circles and contracting them. + +It is impossible to observe this motion successfully, without the aid +of the coloured disc placed near the upper end of the axis. This disc +is divided into sectors, and strongly coloured, so that each sector +may be recognised by its colour when in rapid motion. If the axis +about which the top is really revolving, falls within this disc, its +position may be ascertained by the colour of the spot at the centre of +motion. If the central spot appears red, we know that the invariable +axis at that instant passes through the red part of the disc. + +In this way we can trace the motion of the invariable axis in the +revolving body, and we find that the path which it describes upon the +disc may be a circle, an ellipse, an hyperbola, or a straight line, +according to the arrangement of the instrument. + +In the case in which the invariable axis coincides at first with the +axle of the top, and returns to it after separating from it for a +time, its true path is a circle or an ellipse having the axle in its +\emph{circumference}. The true principal axis is at the centre of the +closed curve. It must be made to coincide with the axle by adjusting +the vertical screws $l$, $m$, $n$. + +Suppose that the colour of the centre of motion, when farthest from +the axle, indicated that the axis of rotation passed through the +sector $L$, then the principal axis must also lie in that sector at +half the distance from the axle. + +If this principal axis be that of \emph{greatest} moment of inertia, +we must \emph{raise} the screw $l$ in order to bring it nearer the +axle $A$. If it be the axis of least moment we must \emph{lower} the +screw $l$. In this way we may make the principal axis coincide with +the axle. Let us suppose that the principal axis is that of greatest +moment of inertia, and that we have made it coincide with the axle of +the instrument. Let us also suppose that the moments of inertia about +the other axes are equal, and very little less than that about the +axle. Let the top be spun about the axle and then receive a +disturbance which causes it to spin about some other axis. The +instantaneous axis will not remain at rest either in space or in the +body. In space it will describe a right cone, completing a revolution +in somewhat less than the time of revolution of the top. In the body +it will describe another cone of larger angle in a period which is +longer as the difference of axes of the body is smaller. The +invariable axis will be fixed in space, and describe a cone in the +body. + +The relation of the different motions may be understood from the +following illustration. Take a hoop and make it revolve about a stick +which remains at rest and touches the inside of the hoop. The section +of the stick represents the path of the instantaneous axis in space, +the hoop that of the same axis in the body, and the axis of the stick +the invariable axis. The point of contact represents the pole of the +instantaneous axis itself, travelling many times round the stick +before it gets once round the hoop. It is easy to see that the +direction in which the hoop moves round the stick, so that if the top +be spinning in the direction $L$, $M$, $N$, the colours will appear in +the same order. + +By screwing the bob B up the axle, the difference of the axes of +inertia may be diminished, and the time of a complete revolution of +the invariable axis in the body increased. By observing the number of +revolutions of the top in a complete cycle of colours of the +invariable axis, we may determine the ratio of the moments of inertia. + +By screwing the bob up farther, we may make the axle the principal +axis of \emph{least} moment of inertia. + +The motion of the instantaneous axis will then be that of the point of +contact of the stick with the \emph{outside} of the hoop rolling on +it. The order of colours will be $N$, $M$, $L$, if the top be +spinning in the direction $L$, $M$, $N$, and the more the bob is +screwed up, the more rapidly will the colours change, till it ceases +to be possible to make the observations correctly. + +In calculating the dimensions of the parts of the instrument, it is +necessary to provide for the exhibition of the instrument with its +axle either the greatest or the least axis of inertia. The dimensions +and weights of the parts of the top which I have found most suitable, +are given in a note at the end of this paper. + +Now let us make the axes of inertia in the plane of the ring unequal. +We may do this by screwing the balance screws $x$ and $x^1$ farther +from the axle without altering the centre of gravity. + +Let us suppose the bob $B$ screwed up so as to make the axle the axis +of least inertia. Then the mean axis is parallel to $xx^1$, and the +greatest is at right angles to $xx^1$ in the horizontal plane. The +path of the invariable axis on the disc is no longer a circle but an +ellipse, concentric with the disc, and having its major axis parallel +to the mean axis $xx^1$. + +The smaller the difference between the moment of inertia about the +axle and about the mean axis, the more eccentric the ellipse will be; +and if, by screwing the bob down, the axle be made the mean axis, the +path of the invariable axis will be no longer a closed curve, but an +hyperbola, so that it will depart altogether from the neighbourhood of +the axle. When the top is in this condition it must be spun gently, +for it is very difficult to manage it when its motion gets more and +more eccentric. + +When the bob is screwed still farther down, the axle becomes the axis +of greatest inertia, and $xx^1$ the least. The major axis of the +ellipse described by the invariable axis will now be perpendicular to +$xx^1$, and the farther the bob is screwed down, the eccentricity of +the ellipse will diminish, and the velocity with which it is described +will increase. + +I have now described all the phenomena presented by a body revolving +freely on its centre of gravity. If we wish to trace the motion of +the invariable axis by means of the coloured sectors, we must make its +motion very slow compared with that of the top. It is necessary, +therefore, to make the moments of inertia about the principal axes +very nearly equal, and in this case a very small change in the +position of any part of the top will greatly derange the +\emph{position} of the principal axis. So that when the top is well +adjusted, a single turn of one of the screws of the ring is sufficient +to make the axle no longer a principal axis, and to set the true axis +at a considerable inclination to the axle of the top. + +All the adjustments must therefore be most carefully arranged, or we +may have the whole apparatus deranged by some eccentricity of +spinning. The method of making the principal axis coincide with the +axle must be studied and practised, or the first attempt at spinning +rapidly may end in the destruction of the top, if not the table on +which it is spun. + +\section*{On the Earth's Motion} + +We must remember that these motions of a body about its centre of +gravity, are \emph{not} illustrations of the theory of the precession +of the Equinoxes. Precession can be illustrated by the apparatus, but +we must arrange it so that the force of gravity acts the part of the +attraction of the sun and moon in producing a force tending to alter +the axis of rotation. This is easily done by bringing the centre of +gravity of the whole a little below the point on which it spins. The +theory of such motions is far more easily comprehended than that which +we have been investigating. + +But the earth is a body whose principal axes are unequal, and from the +phenomena of precession we can determine the ratio of the polar and +equatorial axes of the ``central ellipsoid;'' and supposing the earth to +have been set in motion about any axis except the principal axis, or +to have had its original axis disturbed in any way, its subsequent +motion would be that of the top when the bob is a little below the +critical position. + +The axis of angular momentum would have an invariable position in +space, and would travel with respect to the earth round the axis of +figure with a velocity $\displaystyle = \omega\frac{C - A}{A}$ where +$\omega$ is the sidereal angular velocity of the earth. The apparent +pole of the earth would travel (with respect to the earth) from west +to east round the true pole, completing its circuit in $\displaystyle +\frac{A}{C - A}$ sidereal days, which appears to be about 325.6 solar +days. + +The instantaneous axis would revolve about this axis in space in about +a day, and would always be in a plane with the true axis of the earth +and the axis of angular momentum. The effect of such a motion on the +apparent position of a star would be, that its zenith distance should +be increased and diminished during a period of 325.6 days. This +alteration of zenith distance is the same above and below the pole, so +that the polar distance of the star is unaltered. In fact the method +of finding the pole of the heavens by observations of stars, gives the +pole of the \emph{invariable axis}, which is altered only by external +forces, such as those of the sun and moon. + +There is therefore no change in the apparent polar distance of stars +due to this cause. It is the latitude which varies. The magnitude of +this variation cannot be determined by theory. The periodic time of +the variation may be found approximately from the known dynamical +properties of the earth. The epoch of maximum latitude cannot be +found except by observation, but it must be later in proportion to the +east longitude of the observatory. + +In order to determine the existence of such a variation of latitude, I +have examined the observations of \emph{Polaris} with the Greenwich +Transit Circle in the years 1851-2-3-4. The observations of the upper +transit during each month were collected, and the mean of each month +found. The same was done for the lower transits. The difference of +zenith distance of upper and lower transit is twice the polar distance +of Polaris, and half the sum gives the co-latitude of Greenwich. + +In this way I found the apparent co-latitude of Greenwich for each +month of the four years specified. + +There appeared a very slight indication of a maximum belonging to the +set of months, + +\begin{center} +\begin{tabular}{ccccc} +March, 51. & Feb. 52. & Dec. 52. & Nov. 53. & Sept. 54. +\end{tabular} +\end{center} + +This result, however, is to be regarded as very doubtful, as there did +not appear to be evidence for any variation exceeding half a second of +space, and more observations would be required to establish the +existence of so small a variation at all. + +I therefore conclude that the earth has been for a long time revolving +about an axis very near to the axis of figure, if not coinciding with +it. The cause of this near coincidence is either the original +softness of the earth, or the present fluidity of its interior. The +axes of the earth are so nearly equal, that a considerable elevation +of a tract of country might produce a deviation of the principal axis +within the limits of observation, and the only cause which would +restore the uniform motion, would be the action of a fluid which would +gradually diminish the oscillations of latitude. The permanence of +latitude essentially depends on the inequality of the earth's axes, +for if they had been all equal, any alteration of the crust of the +earth would have produced new principal axes, and the axis of rotation +would travel about those axes, altering the latitudes of all places, +and yet not in the least altering the position of the axis of rotation +among the stars. + +Perhaps by a more extensive search and analysis of the observations of +different observatories, the nature of the periodic variation of +latitude, if it exist, may be determined. I am not aware of any +calculations having been made to prove its non-existence, although, on +dynamical grounds, we have every reason to look for some very small +variation having the periodic time of 325.6 days nearly, a period +which is clearly distinguished from any other astronomical cycle, and +therefore easily recognised. + +\section*{Note: Dimensions and Weights of the parts of the Dynamical Top.} + +\begin{tabular}[b]{|p{.73\textwidth}|r|r|} +\hline +Part & \multicolumn{2}{|c|}{Weight} \\ \cline{2 - 3} + & lb. & oz. \\ \hline\hline +\multicolumn{3}{|l|}{\bf I. Body of the top---} \\ \hline +\hspace{3mm}Mean diameter of ring, 4 inches. & & \\ +\hspace{3mm}Section of ring, $\frac{1}{3}$ inch square. & & \\ +\hspace{3mm}The conical portion rises from the upper and +inner edge of the ring, a height of $1\frac{1}{2}$ inches from the base. & & \\ +\hspace{3mm}The whole body of the top weighs &1&7 \\ \hline +\hspace{3mm}Each of the nine adjusting screws has its screw 1 inch long, +and the screw and head together weigh 1 ounce. + The whole weigh & & 9 \\ \hline +\multicolumn{3}{|l|}{\bf II. Axle, \&c.---} \\ \hline +\hspace{3mm}Length of axle 5 inches, of which $\frac{1}{2}$ inch at the +bottom is occupied by the steel point, $3\frac{1}{2}$ inches are brass +with a good screw turned on it, and the remaining inch is of steel, with +a sharp point at the top. The whole weighs & & $1\frac{1}{2}$ \\ \hline +\hspace{3mm}The bob $B$ has a diameter of 1.4 inches, and a +thickness of .4. It weighs && $2\frac{3}{4}$ \\ \hline +\hspace{3mm}The nuts $b$ and $c$, for clamping the bob and the body +of the top on the axle, each weigh $\frac{1}{2}$ oz. && 1 \\ \hline +{\bf Weight of whole top} &2& $5\frac{1}{4}$ \\ \hline +\end{tabular} + +The best arrangement, for general observations, is to have the disc of +card divided into four quadrants, coloured with vermilion, chrome +yellow, emerald green, and ultramarine. These are bright colours, +and, if the vermilion is good, they combine into a grayish tint when +the rotation is about the axle, and burst into brilliant colours when +the axis is disturbed. It is useful to have some concentric circles, +drawn with ink, over the colours, and about 12 radii drawn in strong +pencil lines. It is easy to distinguish the ink from the pencil +lines, as they cross the invariable axis, by their want of lustre. In +this way, the path of the invariable axis may be identified with great +accuracy, and compared with theory. + +\end{document} + +% End of Project Gutenberg etext of On a Dynamical Top...
\ No newline at end of file diff --git a/old/5192-t.tex/5192.txt b/old/5192-t.tex/5192.txt new file mode 100644 index 0000000..0db13b5 --- /dev/null +++ b/old/5192-t.tex/5192.txt @@ -0,0 +1,384 @@ +The Project Gutenberg EBook of On a Dynamical Top, by James Clerk Maxwell +(#2 in our series by James Clerk Maxwell) + +Copyright laws are changing all over the world. Be sure to check the +copyright laws for your country before downloading or redistributing +this or any other Project Gutenberg eBook. + +This header should be the first thing seen when viewing this Project +Gutenberg file. Please do not remove it. Do not change or edit the +header without written permission. + +Please read the "legal small print," and other information about the +eBook and Project Gutenberg at the bottom of this file. Included is +important information about your specific rights and restrictions in +how the file may be used. 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