Induction coil

The spark coil, also known as Rühmkorff coil, is a historic electrical device for inductively generating high voltage pulses and is referred to in older literature as the induction coil. The device was used in particular in the 19th century and early 20th century varied to

• To generate high voltage pulses to about 250 kV,
• To perform electrical discharge processes at a higher energy level ( Previously, only frictional electricity in diverse variations, eg with the Elektrophor, the Wimshurst machine or Van de Graaff generator was available. )
• Comparisons with electrostatic discharge as to make lightning.

Design and operation

The induction coil consists of a transformer with a high transmission ratio, that is, with a few turns of wire thick as the primary winding with many turns as the secondary winding. The current flow through the primary winding is controlled by a so-called hammer Wagner: An electrical contact is magnetically coupled to the transformer core. When the contact is closed, a current builds up through the primary winding. The magnetic field thus produced, in turn, opens the contact, whereby the current flow is interrupted in the primary winding ( in the diagram shown on the right blue). The magnetic field in the transformer core degrades, the contact in Wagner's hammer falls back to its original position and closes the circuit again, so the process begins again.

Upon interruption of the circuit, the current flow drops to zero in a very short time, resulting in a strong change in the magnetic field. According to law of induction, therefore, it comes at the drop of the secondary winding to a very high voltage pulse ( in the diagram is the curve shown in red). The height of the high voltage pulse is given by the transmission ratio and the speed of the current switch-off.

Parallel to the contact of Wagner's hammer and a capacitor is used to optimize connected, in the form of a Snubbernetzwerkes, on the one hand reduces the spark and the other part forms a resonant circuit with the inductance of the primary coil, which has the same resonance frequency as the secondary coil together with their parasitic capacitance which. In this manner, a resonant transformer is formed which optimize the energy transfer from the primary to the secondary circuit. Parallel to the power source capacitors are used to keep the current pulses occurring in -closing switch of the power supply.

The Wagnerian hammer is suitable neither for greater performance because the switch contact it burns in a short time, nor can thus be achieved at low power switching frequencies above 200 Hz. For higher powers electrolytic Wehnelt interrupter or mercury switches were used, which were separated from the magnetic circuit of the transformer, and switching frequencies allowed up to a few kHz. In this configuration, the function of the induction coil to the first wireless transmitting means in the form of the deletion is about radio stations.

History

The historical development is characterized by a plurality of parallel and individual windings of details of the apparatus. The first spark coil was developed by the Irish clergyman and scientist Nicholas Callan at St Patrick 's College in Maynooth on work done by Michael Faraday in 1836. The design did not correspond to the later standard cylindrical design, but was designed in the shape of a horseshoe. An improvement represented a year later developed by William Sturgeon spark coil, which took advantage of a gear as a switching contact, which had to be operated by hand and so the electrical circuit interrupted periodically.

The electrical contact in the form of Wagner's hammer was developed in 1838 by the Irish James William MacGauley and independently in 1839 by a German, Johann Philipp Wagner. In 1853 the Frenchman Hippolyte Fizeau invented the improvement of the switching contact connected in parallel capacitor, known today as the Snubbernetzwerks. Daniel Henry Rühmkorff succeeded in construction of the secondary winding by an extension in order to improve so as to generate voltage pulses to the 100 kV, from a battery 5 V DC voltage with about 10 km length of wire. Rühmkorff put his spark coil for the first time at the international industrial exhibition in Paris in 1855.

Spark coil by William Sturgeon, 1837, with a gear for controlling the interrupter contact

Spark coil by Charles G. Page, 1838, which uses a bowl of mercury and therein a metal needle as an electrical contact breaker

Spark coil of Heinrich Daniel Rühmkorff to 1850. Besides the Wagnerian Hammer uses this spark coil also a breaker contact from mercury

One of the world's largest radio inductors, built in 1877 by Alfred Apps. With over 350 km of wire length peak voltages could be achieved by the 1 MV.

Application

In addition to scientific applications by physicists induction coils were used in the 19th century popular entertainment at fairs as well as children's toys in the form of electrical machines.

Furthermore fed induction coils the first transmission systems - with their high voltage, a resonant circuit or antenna capacitance charged until the ignition voltage of lying in a circle or the antenna spark gap was reached. The abrupt ignition spark gap suddenly seemed like a closed switch, over which the charge could be offset and oscillatory generated so damped high-frequency oscillations in resonant circuits and antennas. Heinrich Hertz already used a spark coil as the high-voltage generator for its detection of the electromagnetic waves, also called radio wave because of its original type of production.

Induction coils are still part of the facilities of physics teaching in schools, colleges and universities, but are due to the bremsstrahlung ( at these voltages in the form of X-ray radiation) on the anode is released when the electrons strike, rarely used. They serve as a demonstration of discharges of high voltages, for example, the supply of Geissler tubes.

The development path of the induction coil led to the equally constructed coils of gasoline engines, as they are still used today. The first used there instead of Wagner's hammer mechanical Breaker has now been replaced by transistor switches.