Center For Integrating Research and Learning

Arrow1840 - 1849


In this decade scientists sought to deepen their understanding of how electricity and magnetism work and interrelate. Joule’s law, formulated by English physicist James Prescott Joule, added another piece to the puzzle by explaining the relationship between the flow of current through a resistor and the heat given off.

Wheatstone Bridge

Chemist Michael Faraday continued his legendary theoretical work, laboring at a remarkable pace. In 1845, for example, he discovered what became known as the Faraday effect (in which the plane of polarization of light traveling through glass is affected by magnetic lines of force, indicating that magnetism and light are related), considered by many to be his greatest contribution to science. He also identified a new form of magnetism he called diamagnetism (in which substances such as glass are weakly repelled by a magnet). It had been previously believed that magnetic properties were found only in a few elements, such as iron. Faraday recognized that it was a property shared, in varying degrees, by all matter. This insight would later be mined in the work of physicists William Thomson (later known as Lord Kelvin) and James Clerk Maxwell.

Other scientists applying their intellect to theoretical questions included German physicist Wilhelm Weber, who attempted (unsuccessfully) to bring together all existing knowledge on electromagnetism into a single theory (the unit of measure for magnetic flux is named for Weber). His compatriot Hermann von Helmholtz, among other great achievements, developed a statement on the conservation of energy in all its forms, including electrostatic and magnetic.


A great historical step in applied electromagnetism was taken in May 1844, when Samuel Morse sent the first message on a newly completed, federally subsidized telegraph line between Washington, DC, and Baltimore, MD. In Washington, Morse pressed down on the telegraph key, which struck a metal plate and completed an electric circuit. That electricity then flowed through electrical wires strung along poles to a receiver in Baltimore. There, the current flowed through an electromagnet, creating a magnetic field that caused the receiver’s key to be attracted to the plate beneath it. As the key struck the plate, it banged out, in Morse code, a Bible verse that read, “What hath God wrought!”

Scientists’ growing understanding of electricity and how to manipulate it was demonstrated during this period in a measuring device popularized by and named after Charles Wheatstone. Made up of four resistors, a battery and a galvanometer, the Wheatstone bridge measured an unknown electrical resistance by balancing two legs of a bridge circuit.

1840 - 1849


English physicist James Prescott Joule publishes a paper, On the Production of Heat by Voltaic Electricity, in which he describes the amount of heat generated by an electric current (Joule’s law).


Inventor Frederick de Moleyns of England is granted the first patent for an incandescent lamp.


English physicist Charles Wheatstone popularizes an instrument for comparing resistances that came to be known as the Wheatstone bridge, although it was invented by Samuel Christie.


The first official electric telegraph line, constructed with funds appropriated by Congress, is completed in the United States and the initial message is sent by its inventor, Samuel Morse.


German physicist Gustav Kirchhoff introduces his laws of electric circuits, which have since been named in his honor.


British chemist Michael Faraday observes that the plane of polarization of light traveling through glass is affected by magnetic lines of force, a clear indication that magnetism and light are related. The phenomenon produced experimentally by Faraday is referred to as the Faraday effect or Faraday rotation.


Michael Faraday discovers a previously unrecognized form of magnetism in bismuth, glass and a number of other materials that he dubs diamagnetism.


Physicist and mathematician Franz Neumann of Germany publishes his deductions of the mathematical laws for induction of electric currents.


Michael Faraday suggests in a short essay that light could be an electromagnetic phenomenon.


German physicist Wilhelm Weber attempts to unify the analysis and experimental results of André-Marie Ampère, Michael Faraday and others in his development of an electromagnetic theory that involves forces between charged particles in motion. Though his theory is later discounted, Weber’s work would precede many other advances in the field of electromagnetic theory.


Wilhelm Weber puts forward the idea that diamagnetism is simply an example of Faraday’s law impinging upon molecular circuits and suggests that diamagnetism exists in paramagnetic and ferromagnetic substances but is masked due to the comparative strength of the permanent molecular currents the possess.


Hermann von Helmholtz, a German physicist and physician, reads his paper On the Conservation of Force to the Physical Society of Berlin, providing one of the earliest and clearest accounts of the principle of the conservation of energy that governs electrostatic, magnetic, chemical and all other forms of energy.

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