Edison and the Electric Chair_ A Story of Light and Death - Mark Essig [5]
DAVY HAD INVENTED what came to be known as the arc light. At the time it had few practical applications, since batteries powerful enough to produce the effect consumed large amounts of rare metals-silver, copper, zinc—and were therefore enormously expensive. Around 1830, however, scientists discovered a new way to produce electricity. Michael Faraday, who started his scientific career as Davy's assistant, became intrigued by a report that an electric current caused movement in a nearby compass needle. This suggested that electricity produced magnetism. Faraday wondered if the reverse was true—whether magnetism could produce electricity. In 1831 he showed that rotating a coil of conducting wire within the lines of force of a magnetic field caused a current to flow in the wire. Following Farada/s lead, instrument makers in France created the first magneto-electric generators (often shortened to magnetos), hand-cranked machines that spun coils of wire relative to magnetic fields, creating electrical current.
The coils of conducting wire in a generator were known as an armature. As figure 1 shows, when the armature was in the first half of its rotation, the current moved along the conductor in one direction, from point A to point B. But in the second half of the turn, the relationship between the coil and the north and south poles of the magnet was reversed, causing the current to flow from point B to point A. For every 360-degree turn of the coil, the current changed direction twice: from A-B to B-A, then back again. This became known as intermittent—or alternating—current.
Figure 1: First half of rotation (left): When part of the armature cuts the magnetic lines of force near the magnet's north pole, current moves up the wire and produces a positive charge at the lower slip ring. The current is transferred from the slip rings through the brushes and flows through the outside circuit in a clockwise direction. Second half of rotation (right): The same part of the armature now cuts the lines of force near the south pole, causing current to move down the wire and producing a negative charge at the lower slip ring, reversing the current flow. The frequency of current reversal depends on the speed at which the coil rotates.
Electricity so produced behaved differently from battery current, which flowed continuously in one direction. Electrochemistry-decomposing water or isolating sodium from soda, for instance-depended on one electrode remaining positive and the other negative. The same was true for electroplating, in which a brass object such as a spoon was placed in a solution of potassium cyanide in which gold had been dissolved. When an electric current was run through the solution, the spoon—which served as the negative electrode—became coated with a layer of gold. Electroplating and electrochemistry required continuous current, because the processes did not work if each electrode was alternately positive and negative, as was the case with alternating current. Magnetos created a form of electricity that appeared to be unusable.
To solve this problem, instrument makers developed a way to transform the alternating current from a generator into continuous—or direct—current, like that from a battery. This change was accomplished with a switching device called a commutator, which kept the current in the outside circuit flowing in one direction only.12
Direct-current generators proved useful for laboratory demonstrations and electroplating, but the one large mid-nineteenth-century industry that relied on electricity—telegraphy—stuck with batteries, which provided a steadier current. In the late 1830s electric telegraph systems were developed in England by W. F. Cooke and Charles Wheatstone and in the United States by Samuel F. B. Morse.