What is a Jacob's Ladder?
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A Jacob's ladder (more formally, a high voltage traveling arc) is a device for producing a continuous train of large sparks which rise upwards. The spark gap is formed by two wires, approximately vertical but gradually diverging away from each other towards the top.
When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an electric arc. Air breaks down at about 30kV/cm, depending on humidity, temperature, etc. Apart from the anode and cathode voltage drops, the arc behaves almost as a short circuit, drawing as much current as the electrical power supply can deliver, and the heavy load dramatically reduces the voltage across the gap.
The heated, ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more unstable, finally breaking. The voltage across the electrodes then rises and the spark re-forms at the bottom of the device.
This cycle leads to an exotic-looking display of electric white, yellow, blue or purple arcs which is often seen in movies about mad scientists. The device was a staple in schools and science fairs of the 1950s and 1960s, typically constructed out of a Model T spark coil, or any other source of high voltage in the 10,000 volt - 30,000 volt range, like a neon sign transformer or circuit (10-30 kV) or a television picture tube circuit (flyback transformer) (10-28 kV), and two coat hangers or rods built into a "V" shape. For larger ladders, microwave oven transformers connected in series or utility pole transformers (pole pigs) run in reverse (step-up) are used. The sparks can burn through thin paper and plastic and start fires; contact with the exposed high voltage can be lethal.
- Wikipedia, "Spark gap"
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How does it work?
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A Jacob's ladder (more formally, a high voltage traveling arc) is a device for producing a continuous train of large sparks which rise upwards. The spark gap is formed by two wires, approximately vertical but gradually diverging away from each other towards the top. When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an electric arc. Air breaks down at about 30kV/cm, depending on humidity, temperature, etc. Apart from the anode and cathode voltage drops, the arc behaves almost as a short circuit, drawing as much current as the electrical power supply can deliver, and the heavy load dramatically reduces the voltage across the gap. The heated, ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more unstable, finally breaking. The voltage across the electrodes then rises and the spark re-forms at the bottom of the device. This cycle leads to an exotic-looking display of electric white, yellow, blue or purple arcs which is often seen in movies about mad scientists. The device was a staple in schools and science fairs of the 1950s and 1960s, typically constructed out of a Model T spark coil, or any other source of high voltage in the 10,000 volt - 30,000 volt range, like a neon sign transformer or circuit (10-30 kV) or a television picture tube circuit (flyback transformer) (10-28 kV), and two coat hangers or rods built into a "V" shape. For larger ladders, microwave oven transformers connected in series or utility pole transformers (pole pigs) run in reverse (step-up) are used. The sparks can burn through thin paper and plastic and start fires; contact with the exposed high voltage can be lethal. - Wikipedia, "Spark gap" |
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A Jacob's Ladder works because a transformer steps up the voltage coming from the wall. A transformer is an electrical component that exploits Electro-Magnetic Fields and their properties. Whenever an electrical current flows through a wire, that current creates a magnetic field. If the wire is positioned in certain ways, such as in a coil, it will effect the magnetic field. The magnetic field generated by a straight wire is not very strong, so to make a strong field, a wire can be wrapped in a coil; this allows all of the weak fields around wrappings to form into one strong field. Conversely to the current-makes-field phenomenon, a moving magnetic field will also generate a current in a wire. As well, coiling the wire can increase the resulting field.
In a step up or step down transformer, AC wall current generates a field in the primary coil which generates a current in the secondary coil. If different numbers of turnings are used for the two coils, the output voltage and current will be different from the input values. The ratio of the numbers of turns in the two coils determines the value of the step up or down. For example, my transformer has an input of 120V, and it outputs 15kV. 120:15,000 can be reduced to 1:125, the step up ratio of my transformer. This means that the output current will be the input divided by 125, the output voltage will be 125 times greater than the input (120 vs 15,000), and the number of windings on the secondary will be 125 times greater than the number of turnings of the primary.