Fig. 14. Receiving Set.
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. Receiving Set.
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.