Scientific American, April 2, 1898, pages 213-214:
WIRELESS TELEGRAPHY.
At the present moment, when such strained relations exist between Spain and this country, nothing could be more welcome than the announcement of a practical method of carrying on electrical communication between distant points on land, and between ships at sea, without any prearranged connection of any kind between the two points. Many years ago it was found possible to transmit signals through space at a very short range by means of electrical vibrations, but not until the spring of last year had anything of much practical value been accomplished in this direction, with the exception, perhaps, of the method of telegraphing from moving trains which was patented in this country in 1881, and used for limited period on short sections of two of our eastern railroads. During last year Guglielmo Marconi, an Italian student, devoted considerable time to the development of a system of wireless telegraphy, and although he has made use of well known principles, he has so arranged and designed his instruments that he has found it possible to transmit intelligible Morse signals to a distance of over ten miles. It has been left, however, for the American inventor to design apparatus suitable to the requirements of wireless telegraphy in this country. After months of experimenting Mr. W. J. Clarke, of the United States Electrical Supply Company, of this city, has designed, and his company is placing upon the market, such a complete set of wireless telegraphy apparatus that it will in all probability come rapidly into use. For the information of our readers, we illustrate the various pieces of apparatus used, and also explain, with the aid of diagrams, its internal construction and method of operation.
By reference to the diagrams it will be seen that both the transmitting and receiving stations are shown, station A being the transmitting and stations B and C the receiving. The transmitter shown at station A consists of an induction coil, A, specially constructed so as to give the most efficient kind of spark for the purpose. The coil is fitted with an ordinary vibrating make and break, constructed so as to give just the requisite number of interruptions. A special Morse key, B, is placed in the primary circuit, and the condenser is so connected as to kill the spark at the key contact as well as at the vibrating contact. Mounted on the upper part of the coil are three solid brass balls, C, the center one being stationary, and the outside ones adjustable, so that their distance from the center ball can be regulated at will. The two outside balls are connected to the terminals of the secondary coil, as are also the binding posts shown at the side of the coil. It will now be readily seen that when the key, B, is depressed, sparks will pass between the balls and will immediately cease when the key is released. By means of the two binding posts at the side of the coil, one terminal of the secondary coil is connected to earth, and the other terminal to the large metallic plate, C, which should be placed high in the air. The coil may be operated by any suitable battery, but a small storage battery is very much to be preferred.
The receiver at station B consists of two separate instruments, the Clarke coherer relay being mounted on one base, and the polarized receiving relay and sounder upon another. The coherer, G, is a small glass tube made of selected glass, and carefully fitted with two metallic plugs, whose distance from each other in the tube can be readily and accurately adjusted by means of the screw and spring adjustments shown at each end of the tube. The space in the tube between the plugs is partly filled with specially prepared metallic powder, and the two plugs are connected to the binding posts shown, through the small choking coils, 5. These posts are connected to the magnets, L, of the receiving relay through the main battery, J, and binding posts of polarized receiving relay as shown.
One terminal plug of the coherer, G, is connected to earth as shown, and the other terminal plug is connected to the large metallic plate, C, which like the plate at the transmitting station should be placed high in the air. When the powder between the plugs in the tube is lying in its normal condition its resistance is extremely high, often reaching 20,000 ohms, but when the key of the transmitter at the distant station is depressed, electric waves are sent out into space; these waves travel from the plate, C, of the transmitter to the plate, C, of the receiver, and finally reach the powder in the tube, G. Under the action of the waves, the particles of powder in the tube immediately cohere, and their resistance instantly drops down to between 7 and 25 ohms, which great decrease in resistance permits the current from the battery, J, to pass through the circuit, and energize the magnets, L, of the polarized receiving relay, which in turn operates the sounder, N, using the large local battery, K. When the powder in the tube once coheres, it remains in that state until the tube receives a sharp tap, when the powder instantly decoheres and its resistance rises again to an extremely high point. In order that Morse signals can be transmitted it is necessary, of course, that the tap on the tube be automatically accomplished. In order to secure this the decohering magnets, D, are provided and placed in multiple with the magnets of the sounder, so that the sounder and decohering apparatus will operate simultaneously; the decohering magnet operates the vibrating hammer as shown, which it will be seen will keep constantly tapping the tube as long as the key at the distant station is depressed, the powder refusing to decohere as long as the waves are passing through it; but the moment that the key at the transmitting station is released, the last tap of the vibrating hammer, F, decoheres the powder, and thus practically opens the circuit of the battery, J. In order that the apparatus may work properly, it is necessary that every part of it be very carefully constructed, and a wide range of adjustments provided; this last is especially true of the decohering apparatus, which must be so arranged that the vibrating hammer can be adjusted to strike the tube with just the necessary strength of blow. It is also found necessary to have all the magnets wound to a very high resistance, and their terminals provided with resistance coils of still higher resistance; and as the sparks produced by the contacts of the polorized receiving relay, and also by the vibrating contacts of the decohering apparatus, send out waves which affect the coherer, these sparks must be almost entirely suppressed by the use of suitable condensers in the bases of the instruments. This set of apparatus is used for the transmission of Morse signals to moderate distances only, but for longer distances it is simply necessary to use a much larger and properly designed induction coil in connection with the transmitter.
It is frequently desirable to dispense entirely with Morse signals, and this is especially true on shipboard or in places where there is much noise and where a much louder signal or a visual signal is required. To meet these requirements a much less expensive set of apparatus has been designed. The transmitter is precisely the same as in the preceding case, but the polarized receiving relay, R, is much smaller and is not provided with as sensitive adjustments, it having been found that for bell signals they are not necessary. The sounder is entirely dispensed with, and is replaced by a high class vibrating bell, shown at P in the diagram of receiving station C. This bell is so arranged that it can be adjusted to work in unison with the vibrations of the decohering apparatus. The Clarke coherer relay in this case is mounted on top of a mahogany box which contains the decohering magnets, resistance coils for bridging the terminals and also condenser for suppressing the spark at the vibrating contact, as fully shown in the diagram at station C. The plugs in the cohering tube, G, are provided with the same adjustment as in the more elaborate set. The working of the apparatus is perfect in every respect. When required, the vibrating bell, P, can be replaced by an incandescent lamp which can be readily turned on and off from the distant station. It is certainly extremely interesting to place the transmitter of this set in one room and the receiver in another and then listen to the vibrating bell ring out loudly in response to every impulse of the waves. No ground connection, however, or air plate is required for either set of apparatus when the distance between the transmitter and the receiver is comparatively short. For the benefit of those who wish to experiment, and perhaps endeavor to build their own apparatus, a simple coherer is provided which is shown in perspective in one of our half tone illustrations and in detail in the lower engraving. The outer binding posts of this coherer are intended to hold two light rods of metal of equal length projecting out on either side. These rods or wings are necessary when it is desired to transmit to any considerable distance without using the earth connection or earth plate.