Wireless Telegraphy - 1910 Travel Guide
Wireless telegraphy is, in theory, closely allied to heliography, or signaling with flashes of light. The light used, however, is produced electrically and is invisible to the naked eye, owing to the fact that it is made up of very long waves, called Hertzian waves, which vibrate too slowly to affect the retina. The eye can only discern waves which make from 4,000 billions to 7,000 billions vibrations per minute.
However, the Hertzian ray resembles light in that it can be reflected by a metallic plate and can be refracted by a prism of pitch, can be brought to a focus with a pitch lens, and may be polarized. Owing to the great length of the Hertzian waves, almost all substances are transparent to them.
The Hertzian waves were discovered by Professor Heinrich Hertz, a young German philosopher, during his experiments with the spark discharge of Leyden jars and of the Ruhmkorff coil in 1886 and 1887.
He found that when a spark leaped the gap between the terminals, electric oscillations took place in these terminals which set up magnetic waves in the surrounding space, capable in turn of setting up similar oscillations in any adjacent conductor lying at an angle to them. The waves were detected by using a "resonator," which was merely a circle or a rectangle of copper wire formed with a gap in one side.
When the induction coil was in operation and the resonator coil was held near the coil, a tiny stream of sparks would leap across the resonator gap. To better understand this phenomenon take as a crude example two vertical rods in a pool of water and on each a float free to slide vertically on the rod.
Now, if one of these floats be moved up and down upon its rod, it produces waves in the water just as the electric oscillation produces waves in the ether. These spread out in all directions and on reaching the other float cause it to oscillate up and down, just as the magnetic waves produce electric oscillations in the resopator.
Without going into a detailed history of the development of wireless telegraphy from Hertz's experiments. it may be stated that the essential difference between the apparatus used by Hertz in his experiments and the several systems now commonly in use lies in the receiver. The transmitter is practically the same. A vertical wire called the antenna is connected to one terminal of the coil, and the other terminal is connected with the earth, the purpose being to increase the electrical capacity of the terminal rods and produce larger waves.
Instead of producing the oscillations by means of an induction coil, they are now ordinarily produced by a dynamo and a step-up transformer except for telegraphing over short distances. But even with these changes we would not be able to telegraph over any appreciable distance if dependent upon the Hertz resonator for receiving a message, for, owing to the fact that the waves spread out in all directions from the transmitting antenna, the receiving antenna is acted upon by a very small proportion of the power expended by the transmitter, and this proportion decreases very rapidly as the distance between the transmitter and the receiver increases.
In order then to detect the rays at long distances, a very sensitive instrument called the "coherer" has been invented. The coherer in its usual form consists of a glass tube with two metal pistons fitted therein between which a quantity of nickel filings is placed. The latter forms an imperfect electrical contact between the pistons, and takes the place of the spark gap in the receiving antenna.
When the oscillations are set up in the antenna by the Hertzian waves, due to their high pres-sure or voltage, they break through the imperfect contact of the coherer, causing the filings therein to cohere or string together and thus produce a much better electric path through the coherer. The action is microscopic and cannot be detected with the naked eye.
However, the coherer, aside from being a part of the antenna circuit, is also made a part of a local battery circuit, which contains a telegraph receiver, and whenever the electric osculations open a good path through the filings for the local circuit, the telegraph instrument will be energized by the local battery only.
In order to break this path after the oscillations have ceased, or, in other words, to cause the filings to decohere, obey are constantly jarred apart by means of the "tapper," which is in reality an electric bell with the gong removed and the clapper striking the coherer tube instead.
Carbon granules may be substituted for metallic filings, and in this case no tapper is necessary, the coherer being self-restoring.
In transmitting messages a telegraph key in the primary circuit of the induction coil is operated according to the usual Morse code, and this causes sparks to leap the spark gap at corresponding intervals. These signals will then be transmitted by the Hertzian waves to the receiving station, where they will be recorded by the telegraph receiver. The coherer is not by any means the only wave detector in use. Every wireless telegraph company has one or more different types of detectors.