Hertzian Waves
When Maxwell published his famous paper on the nature of the electromagnetic field, the portion of the equations predicting electromagnetic waves was hardly noticed. It wasn’t until more than ten years later when Hertz announced his confirmation of Maxwell’s predictions that scientists became interested and an entire new category of apparatus was developed to use in the study of this new phenomena known as “Hertzian waves”.
Creating Hertzian Waves
To create Hertzian waves, all that is needed is a spark. In fact, the earliest reports of wireless phenomenon or “action at a distance” as it was called, date from the 18th century, when sparks were produced by frictional electric machines or Leyden jars (see pages 12 and 16.) To create sparks you need high voltage, and Hertz used a coil similar to the one below in his early experiments.
To create Hertzian waves, all that is needed is a spark. In fact, the earliest reports of wireless phenomenon or “action at a distance” as it was called, date from the 18th century, when sparks were produced by frictional electric machines or Leyden jars (see pages 12 and 16.) To create sparks you need high voltage, and Hertz used a coil similar to the one below in his early experiments.
The coil operates like a large electric buzzer: The interrupter’s contacts, wired in series between the battery and the coil, are closed in its resting state. When power is applied, the coil is activated, which produces a magnetic field in the iron core of the coil. This pulls the interrupter contacts open, breaking the circuit. The falling magnetic field then relaxes its hold, the contacts close, and the entire cycle repeats.
The rising and falling magnetic field of the primary coil induces a high voltage across the secondary. When the voltage is high enough it jumps the gap, creating a spark. During the time the spark is present there is a high frequency, alternating current electromagnetic field radiated from one pole of the spark gap. This pole is generally connected to some form of antenna.
The rising and falling magnetic field of the primary coil induces a high voltage across the secondary. When the voltage is high enough it jumps the gap, creating a spark. During the time the spark is present there is a high frequency, alternating current electromagnetic field radiated from one pole of the spark gap. This pole is generally connected to some form of antenna.
Detecting Hertzian Waves
Heinrich Hertz used a small spark gap called a resonator (see page 99) in his earliest experiments. Professor Augusto Righi at the University of Bologna developed an improved detector (below). But both detectors suffered from the same drawback; they required direct visual observation and had no mechanism for connecting them to a bell or recorder. That drawback disappeared with the development of the Branley coherer, making wireless telegraphy a possibility.
Heinrich Hertz used a small spark gap called a resonator (see page 99) in his earliest experiments. Professor Augusto Righi at the University of Bologna developed an improved detector (below). But both detectors suffered from the same drawback; they required direct visual observation and had no mechanism for connecting them to a bell or recorder. That drawback disappeared with the development of the Branley coherer, making wireless telegraphy a possibility.