The arc transmitter is an instrument for converting direct current into Radio Frequency or (RF) energy. Prior to the development of the arc transmitter, wireless communications were built around the spark-gap transmitter which was inefficient (it wasted bandwidth and used up more energy than required), disruptive (the wide range of frequencies that spark-gap transmitters generated caused massive interference to other radio frequencies), and unstable.

The arc transmitter (and later, the high-frequency alternator) was able to focus radio energy along a single frequency (or at least a very narrow frequency range which could be easily adjusted to the correct or required frequency). The arc transmitter also used energy more efficiently and allowed wireless audio transmissions, unlike the spark-gap transmitters which were primarily limited to Morse code (dot-dash) communications.

How the Arc Transmitter Came to Be

The arc transmitter was based upon an idea by British engineer William Duddell who was able to use a carbon arc lamp (a lamp containing two electrodes separated by carbon gas which is ionized after the application of an initial breakdown voltage) to make a resonant circuit that produced frequencies in the audal or audible range. Duddell, however, believed that his 'musical arc' would not be able to produce enough oscillation to generate frequencies in the radio frequency (RF) range.

Credit for arc transmitters is usually given to Valdemar Poulsen (the inventor of the first-ever magnetic recording apparatus) who made his own study and performed his own experiments on arc transmission and radio wave generation. He succeeded and was able to raise the frequency to 200,000 cycles per second (which falls within the low-frequency band of radio waves.

The Components of an Arc Transmitter

The working models of arc transmitters used by the marines and shipping companies were built using a chamber made of bronze and cooled by a water cooling system. The chamber was filled with hydrogen. Inside the chamber, there were two electrodes. One of these electrodes was made of copper and the other was made of carbon.

Outside, one coil each can be found above and underneath the chamber; each coil is wrapped around one of the two poles which pierces the chamber on each side of the arc. These coils acted as a magnetic field stabilizer and therefore increased RF wave generation. The direct current of the required voltage was supplied to the chamber by a motor generator. Changing the frequency of transmission was accomplished through the manipulation of the antenna coil.

How an Arc Transmitter Works

Once direct current was applied towards the anode, a negative or electric charge would begin to build. Once the minimum required voltage is breached, the anode will release a spark which would ignite or ionize the hydrogen gas in the chamber. The ionized gas would conduct the electromagnetic wave produced by the breakthrough pulse to the antenna for transmission until the charge becomes too weak to sustain the flow of the electromagnetic wave. The result of the process is radio signals with fixed wave amplitudes and frequencies.