It is generally the receiving apparatus which first attracts the attention of the amateur operator, and so it will be considered first here. An efficient receiving set consists of some form of Detector, Tuning Coil, Telephone Receivers, and Condenser.

Other accessories such as adjustable condensers, potentiometer, battery and testing buzzer improve the outfit and make it more complete.

The choice of the type of instruments must be left entirely to the person who is constructing them. His resources will determine whether he is to use 1,000 ohm telephone receivers built especially for wireless work or ordinary ones having a resistance of only 75 ohms. It is therefore best to read carefully the chapters devoted to the different pieces of receiving apparatus and select the type of detector, tuning coil, etc., which it is desirable to use before commencing the construction of any.

For beginners, I would recommend an outfit consisting of a silicon detector, a double slide tuning coil, a condenser of fixed capacity and 75-ohm telephone receivers. Such an outfit with a 50-foot aerial will receive messages about 150 miles. If 1,000 ohm telephone receivers are used, messages may be read up to 400 miles. Much depends upon the location of the station and the ability of the operator.

A more elaborate and efficient set consists of an electrolytic or "Perikon" detector, a transforming tuner, two adjustable condensers, a potentiometer and a pair of 1,000 ohm telephone receivers. This outfit and a 75-foot aerial could be made to receive 500 to 1,000 miles by a careful operator.

Several cuts of wireless apparatus built for private installation are shown both in this chapter and further through the book, to give an idea of how the better instruments of this type are constructed and finished.

Fig. 10 illustrates a selective receiving set built by the Long Distance Wireless Company. The set is mounted on a mahogany base and the instruments are finished in polished hard rubber and lacquered brass. They comprise a detector stand so arranged that any of the sensitive minerals used in wireless work may be used. The tuning coil is of the double slide type. The condenser is sealed up in a square lacquered brass tube fitted with hard rubber ends and binding posts. The arrangement and construction of the outfit may be readily understood from the cut.

Fig. 11 illustrates a receiving set manufactured by the Wm. J. Murdock Company. The tuning coil is fitted with hard rubber composition ends and is wound on a special core which is not affected by temperature changes. The detector is of the crystal type and is of rather unique construction, since the small fixed condenser is mounted in the base of the detector itself.

The Clapp-Eastham set in Fig. 12 employs a receiving transformer which makes great selectivity possible. The detector is mounted at the left-hand side of the outfit. A very sensitive mineral called "Ferron" is used in the detector. The fixed condenser is enclosed in a brass tube fitted with hard rubber ends and is located on the front of the base, directly in the center. A variable condenser of the rotary type is placed at the right-hand corner.

The Prague Electric Company manufacture the apparatus shown in Fig. 13. The cabinet is mahogany and is fitted with a hard rubber cover. A fixed condenser and a double slide tuning coil are mounted within the cabinet. The sliders of the tuning coil project through two long slots in the cabinet. A universal detector mounted on top of the cabinet is so designed that any material may be experimented with or tested.

Fig. 14 shows a receiving set built up from apparatus described in this book. The cabinet is 12 x 16 inches and 4 inches deep. The wood should be 1/2 inch thick, and in order to present a good appearance is preferably of mahogany. As mahogany is sometimes very hard to procure and expensive, some may find red birch an excellent substitute. When stained with a mahogany stain it presents a fine appearance.

Varnishing and polishing are wasted time when applied to the average amateur's instruments in view of the rough handling and scratching which they receive. The best plan is to stain the wood with an oil stain and give it a wax finish. An oil stain contains no varnish but is merely coloring matter and oil. A good coat should be applied with a wide brush and the surplus stain immediately wiped off by rubbing the whole surface with a piece of cheesecloth. As soon as the stain is thoroughly dry the wood is waxed.

Cut up some beeswax into fine shreds and place it in a jar. Pour some turpentine over the beeswax and let the mixture stand for five or six hours, giving it an occasional stir. Allow it to stand further if necessary until the wax melts and then add enough turpentine to give the mixture a consistency similar to that of thick cream. Apply the preparation to the wood with a rag, and then rub with a piece of clean cheesecloth until the finish is hard and dry. Waxing produces a gloss which is not so bright as a French polish but yet is more durable and not so easily scratched or marred.

A double slide tuning coil made as described in Chapter XIV is mounted on top of the cabinet in the rear. A "universal" detector is mounted in the center, directly in front of the tuning coil. A tubular condenser of fixed capacity is placed on the left-hand side of the detector, and a potentiometer on the opposite side. A double point switch placed directly in front of the detector enables the potentiometer to be brought into play when a battery is used. Four binding posts are mounted on the front of the cabinet. The ground and aerial are connected to the left-hand pair and the telephone receivers to those on the right hand. The wiring diagram is shown also in Fig. 14. By placing the switch A on contact 1, the potentiometer is brought into use. When on contact 2, the potentiometer is cut out. The switch B must be opened when the detector is not in use so as not to run down the battery.

Fig. 15 shows a receiving set somewhat similar to that shown in Fig. 14 but more elaborate and efficient. The cabinet in this case measures 14 x 22 inches and is 6 inches deep. A loosely coupled or transforming tuning coil is used in place of the closely coupled double slide type. Two variable condensers are mounted on either side of the tuning coil. The detector and potentiometer occupy the space directly in front of the tuning coil, while a fixed condenser is placed at their right. The two switches for breaking the battery circuit and disconnecting the potentiometer are in front of the detector. The aerial, ground and telephone receiver leads are connected to binding posts mounted on the front of the cabinet.

The batteries, in both cases, are placed inside the cabinet. The details and construction of all the separate instruments will be found in the respective chapters as denoted by the titles.

Transmitting Range.—A simple transmitting outfit capable of sending about two miles consists of the necessary batteries, a one-inch spark induction coil, a small zinc spark gap and a key. The connections of such a transmitting outfit are shown in Fig. 4.

If the same coil is used with a transmitting helix and a condenser, the range may be increased from 3 to 5 miles.

A 1 1/2-inch spark induction coil using a condenser and a transmitting helix will send about 10 miles, and a 3-inch coil under the same conditions about 20 miles. A 4 and a 6 inch coil will transmit about 30 and 40 miles respectively.

The one quarter kilowatt transformer with a helix and four two-quart leyden jars or an equivalent condenser will transmit at least 50 miles with a suitable aerial. Used as a one half kilowatt transformer with a helix and eight two-quart leyden jars, it will send about 100 miles.

Of course, as in the case of the receiving outfits, these distances are approximate and depend upon the location of the station, the nature of the ground over which the messages are transmitted, the kind of receptor used at the receiving station and the efficiency of the operator himself.

Fig. 16 illustrates a complete Murdock transmitting and receiving set. The transmitting outfit consists of a 15-watt induction coil giving about a one-inch spark, a sending helix, oscillation condenser, a key and a double pole double throw switch for changing the antenna and ground from the transmitting to the receiving instruments or vice versa. The spark gap is mounted on top of the coil. The receiving outfit is the same as that illustrated in Fig. 11.

A complete wireless station—outside of the aerial consists primarily of a source of electrical energy, a transformer or induction coil for charging the oscillation condenser, an oscillation condenser, a transmitting helix, a key for breaking the primary current, a spark gap, an aerial switch, a hot wire ammeter for tuning the transmitting circuits, a detector, a receiving tuner, auxiliary tuning apparatus such as fixed and variable condensers, a potentiometer and battery, and a pair of telephone receivers with a headband.

Other apparatus such as switches, insulators, anchor gaps, testing buzzers, reactance coils, grounding switches, etc., have been described in various places throughout the book and their use suggested whenever it is of any advantage.

The choice of transmitting instruments, as with the receiving apparatus, is left entirely with the experimenter so that he may suit his ideas and means. Wherever possible the range and power of the instruments have been given and suggestions made as to the other apparatus which should be used in connection with them so that the completed outfit will bear some sense of proportion.

Fig. 17 illustrates the portable wireless telegraph set manufactured by the United Wireless Telegraph Company for army service and exploring expeditions or isolated camps. The aerial and the mast can be unloaded, erected, and all parts be ready for operation in fifteen minutes. The mast is made of interchangeable wooden sections. The current for the transmitter is furnished by a portable storage battery. The whole outfit is capable of furnishing efficient service for distances of 25 to 30 miles.