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for Gravitational constant:

Date Researcher Method Value Error Ref
1798 Cavendish, H. Static torsion balance 6.74 0.033 [1]
cell 1 cell 2 cell 3

Contents

[edit] History of measurment

The gravitational constant is the most difficult to measure of all the fundamental constants. This has been attributed to these causes: (1)The weakness of the gravitational force between ordinary sized objects. (2)All matter creates gravitational fields, and there is no way of 'shielding' the measuring apparatus from extraneous fields created by nearby matter or the apparatus itself, as is possible with electromagnetic measurements. (3)Since gravitation theory hasn't been unified with the theory of other forces, G can't be derived from other fundamental constants that can be measured more accurately.

The

The first accurate measurement of G was the landmark Cavendish experiment devised by John Michell and performed by Henry Cavendish in 1798, 71 years after Newton. It was also the first measurement of gravitational force between laboratory-scale masses. Cavendish used a torsion balance to measure the force between lead balls. His remarkably accurate result stood for 97 years, until the Boys measurement of 1895.

Attempts to measure the gravitational force predate the use of a gravitational constant. Interest in G as a physical constant didn't develop until the late 1800s. The first measurement of gravity that calculated G explicitly seems to be that of Cornu & Baille in 1873.[2] In 1894, C. V. Boys gave a lecture before the Royal Society introducing G to the physicists of Britain, who were obviously unfamiliar with it. He pointed out it's importance as a universal fundamental constant of nature. Before this, G was considered an uninteresting constant of proportionality in Newton's law, and the motivation for measuring the force of gravity was instead to determine the mass of the Earth. This was an important unsolved problem of 18th century astronomy (see [[#GM product|]] below).

Another possible barrier to interest in G was that, prior to the adoption of the metric system around 1873???, weight (force) and mass were measured in the same units. In these systems of units, G comes out as numerically equal to R_{earth}^2/M_{earth}\,, tying the universal gravitational constant, which has nothing fundamentally to do with the Earth, to the Earth's mass.

For these reasons, all gravity researchers before 1873 reported their results as a value for the mean density of the Earth, \rho_{earth}\,. However, these can be converted to an equivalent value of G for comparison purposes:

G = \frac{3 g}{4 \pi R_{earth} \rho_{earth}}\,


[edit] Geographical methods

Before Cavendish, technology did not exist to measure the faint gravitational force between laboratory-scale masses, so the first gravity measurements used as one of the attracting masses large geographical objects, such as mountains. The results were very inaccurate; the uncertainties of estimating the mass of the geographical object from surveyed dimensions and samples of rock doomed them to single digit accuracy, if that. However



During the 1800s, after Cavendish showed it was possible, researchers abandoned geographical methods for the more accurate lab-scale methods below.

[edit] Ordinary balance methods

[edit] Torsion balance methods

for Talk:Cavendish experiment:

Thanks for clarifying your reasons; I should have checked with you before making an assumption about them. I hate acting like a POV fanatic. You and User:Astrochemist have outvoted me on this However, I feel I'm right about this. Briefly, my reasons are:

  • (2)This section is not about what Cavendish 'didn't do'. The derivation here is mathematically equivalent to the analysis in Cavendish's paper. This was originally noted at the top of the section, but the note was removed. Although C didn't write equations, if he had he would have ended up with same equation I did. Perhaps it would be instructive to present C's actual calculations, as you suggest. I decided not to because (a)it would have been much longer, (b)it would have been much more difficult to follow, (c)the modern terminology makes the underlying physics concepts clearer, (d)


  1. ^ Cavendish, H. (1798), 'Experiments to determine the Density of the Earth', in MacKenzie, A.S. ed. Scientific Memoirs Vol.9: The Laws of Gravitation, 1900, American Book Co, p.59-105.
  2. ^ Cornu, A. and Baille, J. B. (1873), Mutual determination of the constant of attraction and the mean density of the earth, C. R. Acad. Sci., Paris Vol. 76, 954-958.






For Electroscope:

An electroscope is an early scientific instrument that is used to detect the presence of electric charge on a body. They were the first type of electrical measuring instrument and played a large role in early electricity research. Pith ball and gold leaf electroscopes are still used in schools to demonstrate electricity, and the Lauritzen electroscope is used in radiation dosimeters.

An electroscope merely detects the presence and polarity of charge, and a specialized electroscope which has a calibrated scale to measure the magnitude of charge is called an electrometer. However these terms now follow historical usage.

[edit] How they work

Electroscopes detect electric charge by the motion of a test object due to the Coulomb electrostatic force. The electric potential or voltage of an object equals its charge divided by its capacitance, so electroscopes can be regarded as crude voltmeters. The accumulation of enough charge to cause observable mechanical effects in electroscopes requires hundreds or thousands of volts, so electroscopes are used with high voltage sources such as static electricity and electrostatic machines. Electroscopes can be divided by their method of operation into 4 types:

  • Attraction: The first crude electroscopes worked by the attraction of an uncharged object toward the charge due to induced polarization (see #Pith ball electroscope below.)
  • Repulsion: In these, two hanging objects are given the same charge. Since like charges repel, they spread apart. These can give a crude indication of the magnitude of the charge by the angle of spread.
  • Differential: The first absolute electrometers, these compared the force of the unknown charge on a hanging object to the force provided by an adjustable voltage source. When the two forces were equal, the voltage source was equal to the potential of the unknown charge.
  • Balances: These electrometers measured the force of the charge on a capacitor plate using a balance and weights.

[edit] History

The first electroscope was the versorium, invented in 1600 by William Gilbert, who also studied magnetism. Made in imitation of a compass, it was a pivoted metal needle, but unmagnetized. In the presence of a charged body, the needle would become a dipole and one end would point to the charge. In 1731, Stephen Gray used a hanging thread to detect charge.

The pith ball electroscope, then called the electrical pendulum, was invented by a British weaver, John Canton, in 1757. It was a light ball of pith hanging by a silk thread from a stand. The uncharged pith ball was attracted to a nearby charge by induced polarization, but also could be charged to determine the polarity of the unknown charge. Alessandro Volta standardized the pith ball to make

  • Versorium
  • Pith ball electroscope
  • Carvalle electroscope
  • Gold leaf electroscope
  • Henley or quadrant electrometer
  • Dry pile electrometer
  • Thompson quadrant electrometer
  • Radiation detection provided a new application for electrometers
  • Curie piezoelectrometer
  • Lauritzen quartz electroscope
  • Radiation dosimeters

[edit] Pith ball electroscope

The pith ball electroscope consists of a small light ball of pith suspended by a nonconductive thread from a hook on an insulated stand.

If a charged object is brought near, the pith ball is attracted to it, demonstrating that uncharged objects are attracted to charges due to induced polarization. When an electric charge is brought near an uncharged pith ball, the equal positive and negative charges in the pith ball separate. The charges of opposite sign move toward the side of the ball facing the nearby charge, and the charges of like sign move toward the side away from the charge. Since the unlike charges are now closer to the object, their attraction is greater than the repulsion of the like charges, so the pith ball will experience a net force toward the charge. In a nonconductive object like a pith ball the electrons are bound to atoms, so this movement of charge is actually on an atomic scale, But since there are so many charges it adds up to a measurable force. This is the reason light objects such as styrofoam pellets and plastic wrap are attracted to static charges.

If the ball touches the charged object, some of it's charge is transferred to the ball. Since the ball now has the same charge as the object, it will be repelled. If the ball is given a charge of known polarity or sign (+ or -), it can be used to detect the polarity of an unknown charge, since it will move away from a charge with the same sign but toward a charge with the opposite sign.

[edit] Gold leaf electroscope

The gold leaf electroscope consists of a vertical metal rod, with a disk or ball electrode at the top for receiving charge, and two thin flexible sheets of gold leaf suspended from the lower end, hanging parallel. To protect the gold leaves from air currents they are enclosed in a glass bottle, usually open at the bottom on a grounded conductive base. The leaves are delicate, and too strong a charge can cause them to tear. Usually there are grounded metal plates or foils on either side of the leaves, which improve the sensitivity of the instrument and act as a protective device. If an excessive charge causes the leaves to spread too far they will touch the plates and discharge. Charges can be detected by the electroscope in two different ways:

  • Contact. If a charged object touches the electrode, charge will flow into the rod and the gold leaves. Since like charges repel, the gold leaves will spread apart. If the charged object is removed, the electroscope will be left with a net charge, and the leaves will stay spread.
  • Induction. If a charged object comes near the electrode without touching, the equal positive and negative charges in the electrode will separate, becoming polarized. The charges of opposite sign will be attracted to the object and concentrate on the electrode end. The charges of like sign will be repelled into the gold leaves, causing them to spread. If the object is removed, the charges in the electroscope will mingle again and the leaves will come together, since the electroscope was not left with a net charge.

If the electrode is touched momentarily by a grounded object (like a finger) while the charged object is nearby, the induced charge in the electrode end will drain away to ground, leaving the charge in the leaf end. When the charged object is moved away, the leaves will relax slightly but remain apart, since the electroscope has been left with a net charge.

The gold leaf electroscope is more sensitive than the pith ball variety. Blowing air on the electrode will often get a measureable response, from the ions in the air.



[edit] Electroscope radiation dosimeters

[edit] References




For X-ray tube

X-ray tubes produce X-rays by bombarding a metal target with a beam of high energy electrons, also called cathode rays. An X-ray tube consists of a glass or ceramic envelope containing a high vacuum, with two electrodes. The electrons are produced by a heated cathode, or filament, by thermionic emission, and are accelerated toward the anode or target by a high voltage, typically 30 to 150 kilovolts (kV), between anode and cathode. On striking the metal target the electrons are decelerated by the electric field of the atoms, releasing X-ray photons. The face of the target is usually slanted so the X-rays are emitted at an angle to the electron beam and pass through the side wall of the tube. In imaging applications, the smaller the source of X-rays the sharper the image; so the cathode is shaped to focus the electron beam onto a small spot, perhaps 1 mm in diameter, on the anode.

The intensity of the X-rays emitted is proportional to the current through the tube, typically in the range 1 mA to 1 A. The wavelength spectrum of the X-rays, and their penetrating ability, is determined by the potential across the electrodes.


The process of generating X-rays is very inefficient; only about 1% of the electron beam's energy is converted to X-rays, and the rest is dissipated as heat in the anode. So the anode is usually large and massive to dissipate the heat, and may have a heat sink attached, or in large tubes may be water-cooled. In addition,


[edit] Energy spectrum of X-rays

The amount of energy each electron has when it strikes the target, in electron-volts is equal to the voltage across the tube. An

The fast moving electrons can interact with the atoms of the target to generate X-rays by two different processes, resulting in a spectrum of X-ray energies with two separate components:

  • Bremsstrahlung, a compound of the German brems for braking, and strahlung for radiation. An electron passing close to a metal nucleus will be attracted to the nucleus's positive charge, causing it's path to bend sharply. The acceleration causes the electron to emit X-ray photons. This process produces a broad range of X-ray wavelengths, up to the cutoff wavelength.








for Introduction to Quantum Mechanics

Quantum mechanics (QM, or quantum theory) is a physical science dealing with the behaviour of matter and energy on the small scale of atoms and subatomic particles. QM also forms the basis for the contemporary understanding of how very large objects such as stars and galaxies, and cosmological events such as the Big Bang, can be analyzed and explained. The relation between QM and the more familiar branch of mechanics, traditional or classical mechanics, is that many of the laws of classical mechanics, such as Newton's laws of motion, are approximations derived from QM laws, that hold only for length scales large compared to the 'quantum' scale, and for objects composed of large numbers of 'quantum' sized particles. Quantum mechanics forms the most complete picture of the be universe at all scales.

The term "quantum mechanics" was coined by Max Born in 1924. Historically, although its predictions proved correct by experiment, QM was controversial, hard to understand, and some of its ideas were seen as paradoxical. This is because objects at small scales behave unlike anything in everyday human experience. It is necessary to learn the behavior of

  • Wave-particle duality - Matter and energy exist in packets (energy ones are called 'quanta') that exhibit behavior of both waves and particles. Although particles and waves are familiar in classical mechanics, in large scale physics they are different things; they have properties that are incompatible (for example, waves are spread out in space, particles are at a particular location; waves exhibit refraction, particles don't). Simply, nothing in the large-scale world has the combination of behaviors that these 'particle-waves' have.
  • Heisenberg uncertainty principle - In QM, it is not possible to measure all the variables of a system to arbitrary precision, as is assumed in classical mechanics and other sciences. Measuring instruments must be considered part of the system being measured. Although the disturbing effect of measurement has long been recognized, in classical physics it was always previously assumed that the disturbance could be made negligible by using more delicate measuring instruments, so in principle it was possible to measure the precise 'position', 'velocity', 'momentum', and 'energy' of things. However Because of this, the sets a limit to
  • Because of the disturbance caused by 'measurement' or 'observation', systems behave differently

For these reasons QM is less intuitive than classical mechanics; it requires a higher level of abstraction, and it is often necessary to do without a complete 'mental picture' of what is going on.

Nevertheless QM has been tested and proven correct, in fact to far greater accuracy and thoroughness than many other sciences. Its importance is hard to exaggerate. QM forms the modern foundations of most physical sciences: chemistry, biology, thermodynamics, optics, particle physics, astrophysics, and cosmology, and has created many new applied sciences: nanotechnology, condensed matter physics, quantum chemistry, structural biology, and semiconductor electronics. Through a century of experimentation and practical application, QM has become an essential and established part of physical science.


QM also forms the basis for the contemporary understanding of how very large objects such as stars and galaxies, and cosmological events such as the Big Bang, can be analyzed and explained. Quantum mechanics is the foundation of several related disciplines including nanotechnology, condensed matter physics, quantum chemistry, structural biology, particle physics, and semiconductor electronics.

The term "quantum mechanics" was first coined by Max Born in 1924. The acceptance by the general physics community of quantum mechanics is due to its accurate prediction of the physical behaviour of systems, including systems where Newtonian mechanics fails. Even general relativity is limited—in ways quantum mechanics is not—for describing systems at the atomic scale or smaller, at very low or very high energies, or at the lowest temperatures. Through a century of experimentation and applied science, quantum mechanical theory has proven to be very successful and practical.