A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce low-current, high frequency alternating-current electricity. Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits. Tesla used these circuits to conduct innovative experiments in electrical lighting, phosphorescence, X-ray generation, high frequency alternating current phenomena and the transmission of electrical energy without wires. Tesla coil circuits were used commercially in sparkgap radio transmitters for wireless telegraphy until the 1920s, in medical equipment such as electrotherapy and violet ray devices. Today, their main usage is for entertainment and educational displays, although small coils are still used as leak detectors for high vacuum systems. A Tesla coil is a radio frequency oscillator that drives an air-core double-tuned resonant transformer to produce high voltages at low currents.

Tesla's original circuits as well as most modern coils use a simple spark gap to excite oscillations in the tuned transformer. More sophisticated designs use transistor or thyristor switches or vacuum tube electronic oscillators to drive the resonant transformer. Tesla coils can produce output voltages from 50 kilovolts to several million volts for large coils; the alternating current output is in the low radio frequency range between 50 kHz and 1 MHz. Although some oscillator-driven coils generate a continuous alternating current, most Tesla coils have a pulsed output; the common spark-excited Tesla coil circuit, shown below, consists of these components: A high voltage supply transformer, to step the AC mains voltage up to a high enough voltage to jump the spark gap. Typical voltages are between 30 kilovolts. A capacitor that forms a tuned circuit with the primary winding L1 of the Tesla transformer A spark gap that acts as a switch in the primary circuit The Tesla coil, an air-core double-tuned resonant transformer, which generates the high output voltage.

Optionally, a capacitive electrode in the form of a smooth metal sphere or torus attached to the secondary terminal of the coil. Its large surface area suppresses premature air breakdown and arc discharges, increasing the Q factor and output voltage; the specialized transformer used in the Tesla coil circuit, called a resonant transformer, oscillation transformer or radio-frequency transformer, functions differently from an ordinary transformer used in AC power circuits. While an ordinary transformer is designed to transfer energy efficiently from primary to secondary winding, the resonant transformer is designed to temporarily store electrical energy; each winding has a capacitance across it and functions as an LC circuit, storing oscillating electrical energy, analogously to the way a tuning fork stores vibrational mechanical energy. The primary coil consisting of a few turns of heavy copper wire or tubing, is connected to a capacitor through the spark gap; the secondary coil consists of many turns of fine wire on a hollow cylindrical form inside the primary.

The secondary is not connected to an actual capacitor, but it functions as an LC circuit, the inductance of resonates with stray capacitance, the sum of the stray parasitic capacitance between the windings of the coil, the capacitance of the toroidal metal electrode attached to the high voltage terminal. The primary and secondary circuits are tuned so they resonate at the same frequency, they have the same resonant frequency; this allows them to exchange energy, so the oscillating current alternates back and forth between the primary and secondary coils. The peculiar design of the coil is dictated by the need to achieve low resistive energy losses at high frequencies, which results in the largest secondary voltages: Ordinary power transformers have an iron core to increase the magnetic coupling between the coils; however at high frequencies an iron core causes energy losses due to eddy currents and hysteresis, so it is not used in the Tesla coil. Ordinary transformers are designed to be "tightly coupled".

Due to the iron core and close proximity of the windings, they have a high mutual inductance, the coupling coefficient is close to unity 0.95 – 1.0, which means all the magnetic field of the primary winding passes through the secondary. The Tesla transformer in contrast is "loosely coupled", the primary winding is larger in diameter and spaced apart from the secondary, so the mutual inductance is lower and the coupling coefficient is only 0.05 to 0.2. This means that only 5% to 20% of the magnetic field of the primary coil passes through the secondary when it is open circuited; the loose coupling slows the exchange of energy between the primary and secondary coils, which allows the oscillating energy to stay in the secondary circuit longer before it returns to the primary and begins dissipating in the spark. Each winding is limited to a single layer of wire, which reduces proximity effect losses; the primary carries high currents. Since high frequency current flows on the surface of conductors due to skin effect, it is made of copper tubing or strip with a large surface area to reduce resistance, its turns are spaced apart, which reduces proximity effect losses and arcing between turns.

The output circuit can have two forms: Unipolar – One end of the secondary winding is connected to a single high voltage terminal, the other end is grounded. This type is used in modern coils designed for entertainment; the primary windin

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This is a list of places of interest in the British county of Essex. See List of places in Essex for a list of settlements in Essex. There are a number of territories which form part of the historic boundaries of Essex but which have since been merged into other counties. However, they still form part of the Essex cultural area and some places of interest within those counties have therefore been included below: Now forms part of Greater London as the London Borough of Barking and Dagenham. Barking Abbey Barking Riverside Becontree Estate Eastbury Manor House Ford Dagenham Valence House Museum Now forms part of Greater London as the London Borough of Havering. Brookside Theatre Gidea Park Havering Museum Liberty Shopping Centre Queen's Theatre Rainham Hall Romford Greyhound Stadium Romford Market Royal Liberty School The Brewery The Diver Upminster Tithe Barn Museum of Nostalgia Upminster Windmill Now forms part of Greater London as the London Borough of Newham. Abbey Mills Pumping Stations Boleyn Ground North Woolwich Old Station Museum Queen Elizabeth Olympic Park Queen's Market Three Mills University of East London Docklands Campus University of East London Stratford Campus Now forms part of Greater London as the London Borough of Redbridge.

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