For the shield use a sheet of copper on the back of the panel and place a sheet of copper between the parts, or better, enclose the variometers and detector and amplifying tubes if you use the latter in sheet copper boxes. When you set up the variometers place them so that their stators are at right angles to each other for otherwise the magnetic lines of force set up by the coils of each one will be mutually inductive and this will make the headphones or loud speaker _howl_. Whatever tendency the receptor has to howl with this arrangement can be overcome by putting in a grid leak of the right resistance and adjusting the condenser.

The Parts and How to Connect Them Up.--For this set you require: (1) one _variocoupler_; (2) two _variometers_; (3) one _.001 microfarad variable condenser_; (4) one _.0005 microfarad variable condenser_; (5) one _2 megohm grid leak resistance_; (6) one _vacuum tube detector_; (7) one _6 volt A battery_; (8) one _200 ohm potentiometer_; (9) one _22-1/2 volt B battery_; (10) one _.001 microfarad fixed condenser_, and (11) one pair of _2,000 ohm headphones_.

To wire up the set begin by connecting the leading-in wire to the fixed end of the primary coil, or _stator_, of the variocoupler, as shown in Fig. 56, and connect one post of the .001 mfd. variable condenser to the stator by soldering a short length of wire to the tap of the latter that gives the longest wave you want to receive. Now connect one end of the secondary coil, or _rotor_, of the variocoupler with one post of the .0005 mfd. variable condenser and the other part to the grid of the detector tube. Connect the other end of the rotor of the variocoupler to one of the posts of the first variometer and the other post of this to one of the terminals of the detector filament.

[Ill.u.s.tration: Fig. 56.--Short Wave Regenerative Receiving Set (two Variometers and two Variable Condensers.)]

Connect this filament terminal with the - or negative electrode of the A battery and the + or positive electrode of this with one post of the rheostat and lead a wire from the other post to the free terminal of the filament. This done shunt the potential around the A battery and connect the sliding contact to the - or zinc pole of the B battery and the + or carbon pole of this to one terminal of the headphones, while the other terminal of this leads to one of the posts of the second variometer, the other post of which is connected to the plate of the detector tube. If you want to add an amplifier tube then connect it to the posts instead of the headphones as described in the foregoing set.

CHAPTER XII

INTERMEDIATE AND LONG WAVE REGENERATIVE RECEIVING SETS

All receiving sets that receive over a range of wave lengths of from 150 meters to 3,000 meters are called _intermediate wave sets_ and all sets that receive wave lengths over a range of anything more than 3,000 meters are called _long wave sets_. The range of intermediate wave receptors is such that they will receive amateur, broadcasting, ship and sh.o.r.e Navy, commercial, Arlington's time and all other stations using _spark telegraph damped waves_ or _arc_ or _vacuum tube telephone continuous waves_ but not _continuous wave telegraph signals_, unless these have been broken up into groups at the transmitting station. To receive continuous wave telegraph signals requires receiving sets of special kind and these will be described in the next chapter.

Intermediate Wave Receiving Sets.--There are two chief schemes employed to increase the range of wave lengths that a set can receive and these are by using: (1) _loading coils_ and _shunt condensers_, and (2) _bank-wound coils_ and _variable condensers_. If you have a short-wave set and plan to receive intermediate waves with it then loading coils and fixed condensers shunted around them affords you the way to do it, but if you prefer to buy a new receptor then the better way is to get one with bank-wound coils and variable condensers; this latter way preserves the electrical balance of the oscillation circuits better, the electrical losses are less and the tuning easier and sharper.

Intermediate Wave Set With Loading Coils.--For this intermediate wave set you can use either of the short-wave sets described in the foregoing chapter. For the loading coils use _honeycomb coils_, or other good compact inductance coils, as shown in Chapter X and having a range of whatever wave length you wish to receive. The following table shows the range of wave length of the various sized coils when used with a variable condenser having a .001 microfarad _capacitance_, the approximate _inductance_ of each coil in _millihenries_ and prices at the present writing:

TABLE OF CHARACTERISTICS OF HONEYCOMB COILS

Approximate Wave Length in Meters in

Millihenries Inductance .001 mfd. Variable Mounted Appx. Air Condenser. on Plug

.040 130-- 375 $1.40

.075 180-- 515 1.40

.15 240-- 730 1.50

.3 330-- 1030 1.50

.6 450-- 1460 1.55

1.3 660-- 2200 1.60

2.3 930-- 2850 1.65

4.5 1300-- 4000 1.70

6.5 1550-- 4800 1.75

11. 2050-- 6300 1.80

20. 3000-- 8500 2.00

40. 4000--12000 2.15

65. 5000--15000 2.35

100. 6200--19000 2.60

125. 7000--21000 3.00

175. 8200--24000 3.50

These and other kinds of compact coils can be bought at electrical supply houses that sell wireless goods. If your aerial is not very high or long you can use loading coils, but to get anything like efficient results with them you must have an aerial of large capacitance and the only way to get this is to put up a high and long one with two or more parallel wires s.p.a.ced a goodly distance apart.

The Parts and How to Connect Them Up.--Get (1) _two honeycomb or other coils_ of the greatest wave length you want to receive, for in order to properly balance the aerial, or primary oscillation circuit, and the closed, or secondary oscillation circuit, you have to tune them to the same wave length; (2) two _.001 mfd. variable condensers_, though fixed condensers will do, and (3) two small _single-throw double-pole knife switches_ mounted on porcelain bases.

To use the loading coils all you have to do is to connect one of them in the aerial above the primary coil of the loose coupler, or variocoupler as shown in the wiring diagram in Fig. 57, then shunt one of the condensers around it and connect one of the switches around this; this switch enables you to cut in or out the loading coil at will. Likewise connect the other loading coil in one side of the closed, or secondary circuit between the variable .0007 mfd. condenser and the secondary coil of the loose coupler or variocoupler as shown in Fig. 53. The other connections are exactly the same as shown in Figs. 44 and 45.

[Ill.u.s.tration: Fig. 57.--Wiring Diagram Showing Fixed Loading Coils for Intermediate Wave Set.]

An Intermediate Wave Set With Variocoupler Inductance Coils.--By using the coil wound on the rotor of the variocoupler as the tickler the coupling between the detector tube circuits and the aerial wire system increases as the set is tuned for greater wave lengths. This scheme makes the control of the regenerative circuit far more stable than it is where an ordinary loose coupled tuning coil is used.

When the variocoupler is adjusted for receiving very long waves the rotor sets at right angles to the stator and, since when it is in this position there is no mutual induction between them, the tickler coil serves as a loading coil for the detector plate oscillation circuit.

Inductance coils for short wave lengths are usually wound in single layers but _bank-wound coils_, as they are called are necessary to get compactness where long wave lengths are to be received. By winding inductance coils with two or more layers the highest inductance values can be obtained with the least resistance. A wiring diagram of a multipoint inductance coil is shown in Fig. 58. You can buy this intermediate wave set a.s.sembled and ready to use or get the parts and connect them up yourself.

[Ill.u.s.tration: Fig. 58.--Wiring Diagram for Intermediate Wave Receptor with one Variocoupler and 12 section Bank-wound Inductance Coil.]

The Parts and How to Connect Them Up.--For this regenerative intermediate wave set get: (1) one _12 section triple bank-wound inductance coil_, (2) one _variometer_, and (3) all the other parts shown in the diagram Fig. 58 except the variocoupler. First connect the free end of the condenser in the aerial to one of the terminals of the stator of the variocoupler; then connect the other terminal of the stator with one of the ends of the bank-wound inductance coil and connect the movable contact of this with the ground.

Next connect a wire to the aerial between the variable condenser and the stator and connect this to one post of a .0005 microfarad fixed condenser, then connect the other post of this with the grid of the detector and shunt a 2 megohm grid leak around it. Connect a wire to the ground wire between the bank-wound inductance coil and the ground proper, i.e., the radiator or water pipe, connect the other end of this to the + electrode of the A battery and connect this end also to one of the terminals of the filament. This done connect the other terminal of the filament to one post of the rheostat and the other post of this to the - or negative side of the A battery.

To the + electrode of the A battery connect the - or zinc pole of the B battery and connect the + or carbon pole of the latter with one post of the fixed .001 microfarad condenser. This done connect one terminal of the tickler coil which is on the rotor of the variometer to the plate of the detector and the other terminal of the tickler to the other post of the .001 condenser and around this shunt your headphones. Or if you want to use one or more amplifying tubes connect the circuit of the first one, see Fig. 45, to the posts on either side of the fixed condenser instead of the headphones.

A Long Wave Receiving Set.--The vivid imagination of Jules Verne never conceived anything so fascinating as the reception of messages without wires sent out by stations half way round the world; and in these days of high power cableless stations on the five continents you can listen-in to the messages and hear what is being sent out by the Lyons, Paris and other French stations, by Great Britain, Italy, Germany and even far off Russia and j.a.pan.

A long wave set for receiving these stations must be able to tune to wave lengths up to 20,000 meters. Differing from the way in which the regenerative action of the short wave sets described in the preceding chapter is secured and which depends on a tickler coil and the coupling action of the detector in this long wave set, [Footnote: All of the short wave and intermediate wave receivers described, are connected up according to the wiring diagram used by the A. H. Grebe Company, Richmond Hill, Long Island, N. Y.] this action is obtained by the use of a tickler coil in the plate circuit which is inductively coupled to the grid circuit and this feeds back the necessary amount of current. This is a very good way to connect up the circuits for the reason that: (1) the wiring is simplified, and (2) it gives a single variable adjustment for the entire range of wave lengths the receptor is intended to cover.

The Parts and How to Connect Them Up.--The two chief features as far as the parts are concerned of this long wave length receiving set are (1) the _variable condensers_, and (2) the _tuning inductance coils_.

The variable condenser used in series with the aerial wire system has 26 plates and is equal to a capacitance of _.0008 mfd._ which is the normal aerial capacitance. The condenser used in the secondary coil circuit has 14 plates and this is equal to a capacitance of _.0004 mfd_.

There are a number of inductance coils and these are arranged so that they can be connected in or cut out and combinations are thus formed which give a high efficiency and yet allow them to be compactly mounted. The inductance coils of the aerial wire system and those of the secondary coil circuit are practically alike. For wave lengths up to 2,200 meters _bank litz-wound coils_ are used and these are wound up in 2, 4 and 6 banks in order to give the proper degree of coupling and inductance values.

Where wave lengths of more than 2,200 meters are to be received _coto-coils_ are used as these are the "last word" in inductance coil design, and are especially adapted for medium as well as long wave lengths. [Footnote: Can be had of the Coto Coil Co., Providence, R. I.]

These various coils are cut in and out by means of two five-point switches which are provided with auxiliary levers and contactors for _dead-ending_ the right amount of the coils. In cutting in coils for increased wave lengths, that is from 10,000 to 20,000 meters, all of the coils of the aerial are connected in series as well as all of the coils of the secondary circuit. The connections for a long wave receptor are shown in the wiring diagram in Fig. 59.

[Ill.u.s.tration: Fig. 59.--Wiring Diagram Showing Long Wave Receptor with Variocouplers and Bank-wound Inductance Coils]