Hermann Helmholtz (1821-1894) flourished during a period of the nineteenth century marked by a rapid growth in scientific knowledge, particularly electrical research. His father, Ferdinand Helmholtz, was a close friend of the philosopher Immanuel Hermann Fichte, and because of this association, Herman Helmholtz was exposed to the ideas of Fichte’s great predecessor, Immanuel Kant, whose notions of perception and subjectivity were critical influences.
Helmholtz turned his attention to many areas – biology, aesthetics and theoretical physics. He wrote on the origin of the solar system and the age of the earth, as well as on color and sound perception, chemical thermodynamics and psychology. Early in his professional life he studied nerve transmission and muscle metabolism, which opened for him the timely subject of conservation of energy, covered in a treatise written in 1847, On the Conservation of Force.
A quarter century later he studied wave oscillations, which he observed by connecting a coil to a Leyden jar. His interest in electromagnetism led him to develop what we call the Helmholtz equation, a partial differential equation, which greatly reduces the complexity of analysis by making use of the technique known as separation of variables.
Besides being a consummate theoretician, Helmholtz was an accomplished builder of instruments. Early in his professional life he achieved renown by devising and building the ophthalmoscope, an instrument permitting examination of the inside of the human eye for medical purposes.
Years later, Helmholtz built what came to be known as the Helmholtz coil.
It actually consists of two identical electrical coils mounted so as to share a common axis. The two coils are separated by a distance that is equal to their radii. Equal amounts of electrical current of the same polarity are fed to the two coils. The region between them is used in various experimental endeavors that require an environment free of a magnetic gradient, because the two magnetic fields cancel one another and mask the earth’s magnetic field.
The Helmholtz coil is still marketed and used today in conjunction with a Hall effect sensor where a homogenous magnetic field is needed. Typical specifications are:
Coil diameter: 295 mm
Coil spacing: 150 mm
Number of turns per coil: 124
Enameled copper wire thickness: 1.5 mm
DC resistance: 1.2 ohms each
Maximum coil current: 5 amp
Maximum coil voltage: 6 volt
Maximum flux density at 5 amp: 3.7 mT
Another instrument this versatile researcher built is known as the Helmholtz resonator. It is an enhanced version of the bottle that can be made to issue a musical note by blowing across the opening. (Flute players, by tightening the embrasure, can play multiple harmonics.)
Helmholtz used his resonator to examine the properties of the resonant air cavity. Its volume in conjunction with the area of the neck and other parameters gives rise to a number of complex formulae which are used to calculate sonic properties. These calculations are used today in the design of high-end sound system speakers, internal combustion engine intake manifolds and high-performance exhaust systems.
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