# Spark inductor

Spark inductor from 1920

The spark inductor , also known as the Rühmkorff coil , is a historical electrical device for the inductive generation of high-voltage pulses and is also known as an inductor in older literature . Before the spark inductor there was only static electricity in the most diverse variations and charges separated by influence, e.g. B. with the electrophore , the Wimshurst machine or the ribbon generator .

The device was used in a variety of ways in particular in the 19th century and the beginning of the 20th century

• Generate high voltage pulses of up to approx. 250  kV ,
• to be able to carry out electrical discharge processes at a higher energy level,
• Compare with electrostatic discharges such as lightning .

## Structure and functionality

Voltage and current curve over time on the spark inductor without capacitor
Voltage and current over time on the spark inductor with capacitor

The spark inductor consists of a transformer with a high transformation ratio, i.e. with a few turns of thick wire as the primary winding and with many turns as the secondary winding. The flow of current through the primary winding is controlled by a so-called Wagner hammer : an electrical contact is magnetically connected to the transformer core . When the contact is closed, a current builds up through the primary winding. The magnetic field generated in this way opens the contact, which interrupts the flow of current in the primary winding (shown in blue in the diagram on the right). The magnetic field in the transformer core dissipates, the contact in Wagner's hammer falls back to its starting position and closes the circuit again, which starts the process again. ${\ displaystyle i_ {1}}$${\ displaystyle i_ {1}}$

When the circuit is interrupted, the current flow drops to zero within a very short time, which leads to a strong change in the magnetic field. According to the law of induction , when the voltage drops in the secondary winding, there is a very high voltage pulse (the curve is shown in red in the diagram). The height of the high voltage pulse is given by the transmission ratio and the speed at which the power is switched off. ${\ displaystyle i_ {1}}$${\ displaystyle u_ {2}}$

In parallel to the contact of the Wagner hammer, a capacitor is also connected for optimization , in the form of a snubber network , which on the one hand reduces spark formation and on the other hand forms an oscillating circuit with the inductance of the primary coil, which has the same resonance frequency as the secondary coil together with its parasitic capacitance. In this way, a resonance transformer is formed which optimizes the transfer of energy from the primary to the secondary circuit. In parallel with the power source, capacitors are used to keep the power surges that occur when the switch closes away from the power supply.

The Wagner hammer is not suitable for higher power, since the switching contact burns down in a short time, nor can switching frequencies of over 200 Hz be achieved with low power. Electrolytic Wehnelt interrupters or mercury switches , which were separated from the magnetic circuit of the transformer and allowed switching frequencies of up to a few kHz, were used for higher outputs . In this configuration, the function of the spark inductor is transferred to the first wireless transmission devices in the form of the extinguishing spark transmitter .

## history

The historical development is characterized by a multitude of parallel and individual developments of details of the apparatus. The first spark inductor was developed in 1836 by the Irish clergyman and scientist Nicholas Callan at St Patrick's College in Maynooth , following preliminary work by Michael Faraday . The design did not correspond to the later usual cylindrical design, but was designed in the shape of a horseshoe. An improvement was the spark inductor developed by William Sturgeon a year later , which used a gear wheel as a switching contact, which had to be driven by hand and thus periodically interrupted the electrical circuit.

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 the German Johann Philipp Wagner . In 1853, the French Hippolyte Fizeau invented the improvement of the capacitor connected in parallel to the switch contact, known today as the Snubber network. Heinrich Daniel Rühmkorff succeeded in improving the structure of the secondary winding by extending it in order to generate voltage impulses around 100 kV from a battery with 5 V direct voltage with a wire length of approx. 10 km. Rühmkorff exhibited its spark inductor for the first time at the international industrial exhibition in Paris in 1855.

## application

Ignition coils from the automotive sector, today's design of the spark inductor

In addition to scientific applications by physicists, spark inductors were used in the 19th century for popular amusement at fairs and as children's toys in the form of electrifying machines.

Furthermore, spark inductors fed the first transmission systems - their high voltage was used to charge an oscillating circuit or antenna capacitance until the ignition voltage of a spark gap in the circle or antenna was reached. The spark gap, ignited abruptly, acted like a suddenly closed switch, via which the charge could oscillate and thus generate damped high-frequency oscillations in resonant circuits and antennas . Already Heinrich Hertz used a spark coil as a high voltage generator for its demonstration of electromagnetic waves because of their original production use radio waves are called.

Spark inductors are still part of the equipment for physics classes in schools, colleges and universities, but are rarely used due to the bremsstrahlung (at these voltages in the form of X-rays ) that is released when the electrons hit the anode . They are used to demonstrate high voltage discharges, e.g. B. also the supply of Geissler tubes .

The development path of the spark inductor led to the ignition coils of gasoline engines , which are also constructed , as they are still used today. The mechanical ignition interrupter initially used there instead of Wagner's hammer has now been replaced by transistor switches.