An electric shock can occur upon contact of a human or animal body with any source of voltage high enough to cause sufficient current flow through the muscles or nerves. The minimum detectable current in humans is thought to be about 1 mA. The current may cause tissue damage or heart fibrillation if it is sufficiently high.
Description of electric shock
An electric shock is usually painful and can be fatal. The level of voltage is not a direct guide to the level of injury or danger of death, despite the common misconception that it is. A small shock from static electricity may contain thousands of volts but has very little current behind it due to high internal resistance. Physiological effects and damage are generally determined by current and duration. Even a low voltage causing a current of extended duration can be fatal. It should be noted, however, that Ohm's Law directly correlates voltage and current for a given resistance; thus, for a particular path through the body under a particular set of conditions, a higher voltage will produce a higher current flow.
'Let go' current
With sufficiently high current there can be a muscular spasm which causes the affected person to grip and be unable to release from the current source. The maximum current that can cause the flexors of the arm to contract but that allows a person to release his hand from the current's source is termed the let-go current. For direct current, the let-go current is about 75 mA for a 70-kg man. For alternating current, the let go current is about 15 mA, dependent on muscle mass.
Invented of electric chair
The first practical electric chair was invented by Harold P. Brown. Brown was an employee of Thomas Edison, hired for the purpose of researching electrocution and for the development of the electric chair. Since Brown worked for Edison, and Edison promoted Brown's work, the development of the electric chair is often erroneously credited to Edison himself. Brown's design was based on George Westinghouse's alternating current, which was then just emerging as the rival to Edison's less transport-efficient direct current, which was further along in commercial development. The decision to use alternating current was entirely driven by Edison's attempt to claim that alternating current was more lethal than direct current.
In order to prove that alternating current electricity was better for executions, Brown and Edison killed many animals, including a circus elephant, while testing out their prototypes. They also held executions of animals for the press in order to ensure that alternating current was associated with electrocution. It was at these events that the term "electrocution" was coined. Edison introduced the verb "to westinghouse" for denoting the art of executing persons with alternating current. Most of their experiments were conducted at Edison's West Orange, New Jersey, laboratory in 1888.
Method of using electric chair
The condemned prisoner is typically strapped into the chair, with one electrode attached to the head and a second attached to the leg. At least two shocks of an electrical current would be applied for several minutes, depending on the person. An initial voltage of around 2,000 volts is used to break the initial resistance of the skin and cause unconsciousness. The voltage is then lowered to reduce current flow so as to prevent burning. A current flow of around 8 amps is usual. The body of the condemned would heat up to 59°C, and the electric current would cause severe damage to internal organs.
In theory, unconsciousness occurs in a fraction of a second. There have been reports of victims' heads on fire, of burning transformers, and of letting the crying victim wait in pain on the floor of the execution room while the chair was fixed. In 1946, the electric chair failed to kill Willie Francis, who reportedly shrieked "Stop it! Let me breathe!" as he was being executed. It turned out that the portable electric chair had been improperly set up by an intoxicated trustee.
Ďaľšie referáty z kategórie
Electric shocks and electric chairs
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