Weber-Kohlrausch experiment

The Weber-Kohlrausch experiment is a historical physics experiment carried out by the physicists Wilhelm Eduard Weber and Friedrich Kohlrausch in 1855. It provided the first clue that light is an electromagnetic phenomenon.

In essence, the experiment relied on measuring the amount of charge on a Leyden bottle in two different ways. First, the amount of charge was measured using electrostatic methods, i.e. by determining the force that electrical charges exert on one another. Then the Leydener bottle was discharged via a coil, whereby the discharge current caused a magnetic field. Its flux density was compared with the flux density of the earth's magnetic field using a compass needle . The strength of the earth's magnetic field had previously been determined experimentally in another way and was assumed to be known. From the flux density of the magnetic field inside the coil, conclusions could finally be drawn about the amount of charge that had flowed and it was also determined electrodynamically .

When comparing the amount of charge measured electrostatically and electrodynamically, it became apparent that a constant was important, which had the dimension of a speed. Weber and Kohlrausch gave a value for this constant that differed by a factor from the actual value, because in their time a faulty model was still used for the transport of electrical current. The value (corrected based on current knowledge) was ${\displaystyle {\sqrt {2}}}$

${\displaystyle c\approx 3{,}1\cdot 10^{8}\,\mathrm {m\,s^{-1}} }$

and agreed within the scope of the measurement accuracy with the value of the vacuum speed of light used at the time . This result inspired James Clerk Maxwell to assume that light is an electromagnetic wave . It follows from Maxwell's equations that electromagnetic waves propagate at a speed that can be calculated using the following equation:

${\displaystyle c={\frac {1}{\sqrt {\mu _{0}\varepsilon _{0}}}}}$

Here are the magnetic field constant and the electric field constant of the vacuum. The identity of the speed of light in a vacuum with the constant from the experiment by Weber and Kohlrausch was given a theoretical justification. By 2019, the three constants , and were precisely defined in the SI system , thus defining the basic units. In the meantime, the unit ampere has been defined differently, which is why the two field constants are again subject to a certain measurement uncertainty. However, since the speed of light is still precisely defined, the product of and continues to be exact. ${\displaystyle \mu _{0}}$ ${\displaystyle \varepsilon _{0}}$ ${\displaystyle c}$ ${\displaystyle \mu _{0}}$ ${\displaystyle \varepsilon _{0}}$ ${\displaystyle \mu _{0}}$ ${\displaystyle \varepsilon _{0}}$

Individual evidence

1. ^ Matthias Elbel: The Kohlrausch / Weber experiment . In: Paths in physics didactics . tape 3 . Erlangen 1993 ( physikverhaben.de [PDF; accessed December 10, 2019]).