temporaldurationsequence of events
Current established physical theories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measured, even in principle. Tentative physical theories that describe this time scale exist; see for instance loop quantum gravity. Time travel is the concept of moving backwards or forwards to different points in time, in a manner analogous to moving through space, and different from the normal "flow" of time to an earthbound observer. In this view, all points in time (including future times) "persist" in some way. Time travel has been a plot device in fiction since the 19th century.

Steelyard balance

A steelyard balance, steelyard, or stilyard is a straight-beam balance with arms of unequal length. It incorporates a counterweight which slides along the longer arm to counterbalance the load and indicate its weight. A steelyard is also known as a Roman steelyard or Roman balance.

Instrument error

If the instrument has a needle which points to a scale graduated in steps of 0.1 units, then depending on the design of the instrument it is usually possible to estimate tenths between the successive marks on the scale, so it should be possible to read off the result to an accuracy of about 0.01 units. The act of taking the measurement may alter the quantity being measured. For example, an ammeter has its own built-in resistance, so if it is connected in series to an electrical circuit, it will slightly reduce the current flowing through the circuit.

Test method

The results of testing can be qualitative (yes/no), quantitative (a measured value), or categorical and can be derived from personal observation or the output of a precision measuring instrument. Usually the test result is the dependent variable, the measured response based on the particular conditions of the test or the level of the independent variable. Some tests, however, may involve changing the independent variable to determine the level at which a certain response occurs: in this case, the test result is the independent variable.

Humphry Davy

Sir Humphry DavyDavySir Humphry Davy, Bt
"It [science] has bestowed on him powers which may almost be called creative; which have enabled him to modify and change the beings surrounding him, and by his experiments to interrogate nature with power, not simply as a scholar, passive and seeking only to understand her operations, but rather as a master, active with his own instruments." The first lecture garnered rave reviews, and by the June lecture Davy wrote to John King that his last lecture had attendance of nearly 500 people. "There was Respiration, Nitrous Oxide, and unbounded Applause. Amen!" Davy revelled in his public status.

Alessandro Volta

VoltaVolta, AlessandroA. Volta
In this way he discovered the electrochemical series, and the law that the electromotive force (emf) of a galvanic cell, consisting of a pair of metal electrodes separated by electrolyte, is the difference between their two electrode potentials (thus, two identical electrodes and a common electrolyte give zero net emf). This may be called Volta's Law of the electrochemical series. In 1800, as the result of a professional disagreement over the galvanic response advocated by Galvani, Volta invented the voltaic pile, an early electric battery, which produced a steady electric current.

Octant (instrument)

octantHadley's quadrantoctants
For example, Spencer, Browning and Rust, a manufacturer of scientific instruments in England from 1787 to 1840 (operating as Spencer, Browning and Co. after 1840) used a Ramsden dividing engine to produce graduated scales in ivory. These were widely used by others and the SBR initials could be found on octants from many other manufacturers. Examples of these very similar octants are in the photos in this article. The image at the top is essentially the same instrument as the one in the detail photos.

Michael Faraday

FaradayFaraday, MichaelM. Faraday
Michael Faraday FRS (22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, diamagnetism and electrolysis. Although Faraday received little formal education, he was one of the most influential scientists in history. It was by his research on the magnetic field around a conductor carrying a direct current that Faraday established the basis for the concept of the electromagnetic field in physics. Faraday also established that magnetism could affect rays of light and that there was an underlying relationship between the two phenomena.


A protractor is a measuring instrument, typically made of transparent plastic or glass, for measuring angles. Most protractors measure angles in degrees . Radian-scale protractors measure angles in radians. Most protractors are divided into 180 equal parts. Some precision protractors further divide degrees into arcminutes. They are used for a variety of mechanical and engineering-related applications. One common use is in geometry lessons in schools. Some protractors are simple half-discs. More advanced protractors, such as the bevel protractor, have one or two swinging arms, which can be used to help measure the angle.

Spirit level

bubble levellevellevels
A similar procedure is applied to more sophisticated instruments such as a surveyor's optical level or a theodolite and is a matter of course each time the instrument is set up. In this latter case, the plane of rotation of the instrument is levelled, along with the spirit level. This is done in two horizontal perpendicular directions. The sensitivity is an important specification for a spirit level; its accuracy depends on its sensitivity. The sensitivity of a level is given as the change of angle or gradient required to move the bubble by unit distance.

Adolphe Quetelet

QueteletLambert Adolphe Jacques QueteletQuetelet, Adolphe
He founded the science of anthropometry and developed the body mass index scale, originally called the Quetelet Index. Adolphe was born in Ghent (which, at the time was a part of the new French Republic). He was the son of François-Augustin-Jacques-Henri Quetelet, a Frenchman and Anne Françoise Vandervelde, a Flemish woman. His father was born at Ham, Picardy, and being of a somewhat adventurous spirit, he crossed the English Channel and became both a British citizen and the secretary of a Scottish nobleman. In that capacity, he traveled with his employer on the Continent, particularly spending time in Italy.


thinnessskinnylow weight
Underweight people have a body mass index (BMI) of under 18.5 or a weight 15% to 20% below that normal for their age and height group. A person may be underweight due to genetics, metabolism, drug use, lack of food (frequently due to poverty), eating disorder, or illness (both physical and mental). Being underweight is associated with certain medical conditions, including anorexia, type 1 diabetes, hyperthyroidism, cancer, or tuberculosis. People with gastrointestinal or liver problems may be unable to absorb nutrients adequately. People with certain eating disorders can also be underweight due to lack of nutrients/over exercise.

Polarization (electrochemistry)

polarizationdepolarizationselectrical polarization
In electrochemistry, polarization is a collective term for certain mechanical side-effects (of an electrochemical process) by which isolating barriers develop at the interface between electrode and electrolyte. These side-effects influence the reaction mechanisms, as well as the chemical kinetics of corrosion and metal deposition. In a reaction we can displace the bonding electrons by attacking reagents. The electronic displacement in turn may be due to certain effects, some of which are permanent (inductive and mesomeric effects), and the others are temporary (electromeric effect).


microammetermoving coil meterampere-meter
Consequently, such meters would normally have a non linear scale, but the iron parts are usually modified in shape to make the scale fairly linear over most of its range. Moving iron instruments indicate the RMS value of any AC waveform applied. Moving iron ammeters are commonly used to measure current in industrial frequency AC circuits. In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current. These also measure true RMS for an applied AC.

Zinc–carbon battery

zinc-carbon batteryzinc-carbon batterieszinc-carbon
A zinc–carbon battery is a dry cell primary battery that delivers about 1.5 volts of direct current from the electrochemical reaction between zinc and manganese dioxide. A carbon rod collects the current from the manganese dioxide electrode, giving the name to the cell. A dry cell is usually made from zinc which serves as the anode with a negative electrical polarity, while the inert carbon rod is the positive electrical pole cathode. General purpose batteries may use an aqueous paste of ammonium chloride as electrolyte, possibly mixed with some zinc chloride solution. Heavy duty types use a paste primarily composed of zinc chloride.

Nernst equation

NernstNernst potential
In electrochemistry, the Nernst equation is an equation that relates the reduction potential of an electrochemical reaction (half-cell or full cell reaction) to the standard electrode potential, temperature, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation. It was named after Walther Nernst, a German physical chemist who formulated the equation. The Nernst equation is derived from the standard changes in the Gibbs free energy associated with an electrochemical transformation.

Walther Nernst

Walther Hermann NernstNernstHermann Walther Nernst
After eighteen productive years at Göttingen, investigating osmotic pressure and electrochemistry and presenting a theory of how nerves conduct, he moved to Berlin, and was awarded the title Geheimrat In 1905, he proposed his "New Heat Theorem", later known as the Third law of thermodynamics. He showed that as the temperature approached absolute zero, the entropy approaches zero — while the free energy remains above zero. This is the work for which he is best remembered, as it enabled chemists to determine free energies (and therefore equilibrium points) of chemical reactions from heat measurements.


measuredtwo-terminal (2T) sensing
Measuring instrument. Electronic test equipment. Electronics. Electric circuit. List of electronics topics. Series and parallel circuits. Galvanometer. Rheochord.

Electrochemical Society

The Electrochemical SocietyAmerican Electrochemical SocietyBulletin of the Electrochemical Society
Norman Hackerman: Known internationally as an expert in metal corrosion, Hackerman is most recognized for developing the electrochemistry of oxidation. Carl Wagner: Often referred to as the father of solid state chemistry, Wagner's work on oxidation rate theory, counter diffusion of ions, and defect chemistry considerably advanced our knowledge of how reactions proceed at the atomic level in the solid state. Charles W. Tobias: As former president of the society and pioneer in the field of electrochemical engineering, Tobias was instrumental in the advancement of electrochemical science.

Electric current

currentelectrical currentcurrents
The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas (sound waves move through air much faster than large-scale motions such as convection). The random motion of charges is analogous to heat – the thermal velocity of randomly vibrating gas particles. Alternating current. Current 3-vector. Direct current. Electric shock. Electrical measurements. History of electrical engineering. Hydraulic analogy. International System of Quantities. SI electromagnetism units. Single-phase electric power. Two-phase electric power. Three-phase electric power.


The earliest evidence of the lever mechanism dates back to the ancient Near East circa 5000 BC, when it was first used in a simple balance scale. In ancient Egypt circa 4400 BC, a foot pedal was used for the earliest horizontal frame loom. In Mesopotamia (modern Iraq) circa 3000 BC, the shadouf, a crane-like device that uses a lever mechanism, was invented. In ancient Egypt technology, workmen used the lever to move and uplift obelisks weighing more than 100 tons. This is evident from the recesses in the large blocks and the handling bosses which could not be used for any purpose other than for levers.


barometricaneroid barometerbarometers
A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs, pressure systems and frontal boundaries. Barometers and pressure altimeters (the most basic and common type of altimeter) are essentially the same instrument, but used for different purposes.


chemistsresearch chemistchemical
The work of chemists is often related to the work of chemical engineers, who are primarily concerned with the proper design, construction and evaluation of the most cost-effective large-scale chemical plants and work closely with industrial chemists on the development of new processes and methods for the commercial-scale manufacture of chemicals and related products. The roots of chemistry can be traced to the phenomenon of burning. Fire was a mystical force that transformed one substance into another and thus was of primary interest to mankind. It was fire that led to the discovery of iron and glasses.

Quantum electrochemistry

The scientific school of Quantum electrochemistry began to form in the 1960s under Revaz Dogonadze. Generally speaking, the field comprises the notions arising in electrodynamics, quantum mechanics, and electrochemistry; and so is studied by a very large array of different professional researchers. The fields they reside in include, chemical, electrical and mechanical engineering, chemistry and physics. More specifically, quantum electrochemistry is the application of quantum mechanical tools such as density functional theory to the study of electrochemical processes, including electron transfer at electrodes. It also includes models such as Marcus theory.

Fritz Haber

HaberF. HaberHaber, Fritz
Discussions with Carl Engler prompted Haber to explain autoxidation in electrochemical terms, differentiating between dry and wet autoxidation. Haber's examinations of the thermodynamics of the reaction of solids confirmed that Faraday's laws hold for the electrolysis of crystalline salts. This work led to a theoretical basis for the glass electrode and the measurement of electrolytic potentials. Haber's work on irreversible and reversible forms of electrochemical reduction are considered classics in the field of electrochemistry. He also studied the passivity of non-rare metals and the effects of electric current on corrosion of metals.