APPARATUS REQUIRED IN MANY INSTANCES IN ORDER TO COMPLY WITH THE WIRELESS LAW.
OSCILLATION HELIX
The oscillation helix has almost become a necessity in order to comply with the regulations of the Wireless Law regarding wave form, except in those stations where a quenched gap is used.
Fig. 151. Amco Oscillation Helix.
The wave emitted by many stations is not pure. It is composed usually of two or three separate waves of different lengths instead of all the energy being confined to oscillations of one period. It is possible to tune such a wave in two or more places or "humps," as they are called, on the tuning coil. It is obvious that a wave possessing such humps cannot be closely tuned and is liable to interfere with the signals of another station. This is one of the principal causes of interference.
The reason for this phenomenon is simple. The action of a transmitter is to first charge a condenser. When the potential of the condenser rises to sufficient value it discharges across the spark gap and sets up oscillations in the closed circuit. These oscillations immediately induce oscillations in the open circuit or aerial system and part of the energy passes off into the ether as electro-magnetic waves. However, the oscillations in the aerial system do not immediately die away after the oscillations in the closed circuit cease during the interim until the next condenser discharge, but continue to surge and react upon the dosed circuit to sufficient extent to induce therein currents which surge back and forth long after the current from the condenser discharge has died away.
We might call the oscillations due to the condenser discharge primary oscillations and those induced in the aerial thereby secondary oscillations. Those which are then set up in the closed circuit by the reaction of the secondary currents are tertiary. This third train of oscillations persist after the secondary currents have died away, and induce another set of oscillations in the aerial which send out a second set of electromagnetic waves differing in length from the first.
The oscillations which take place after the initial surge in the closed and open circuits are naturally somewhat weak. By using an oscillation helix in which the primary and secondary are separated from each other it is possible to eliminate the third and fourth trains of oscillations and all others having a tendency to follow, by placing the circuits apart so that the weak oscillations are not strong enough* to react across the intervening space. The immediate oscillations set up by the condenser discharge are strong enough to act across the space and set up powerful oscillations in the aerial.
Fig. 152. Details of Oscillation Helix Construction.
A hot wire ammeter placed in the aerial circuit of a transmitter employing an oscillation transformer will not indicate as much current as if placed in the same position in a circuit using an ordinary helix; but in spite of the fact, a transmitter using an oscillation helix will send farther because the energy is concentrated in waves of one length.
The construction of one type of oscillation helix has already been outlined on page 92. The form shown in Fig. 152 has no special advantages over the other but is preferred by many experimenters.
It is of the "pancake" type, so-called because of the flat form of the windings which are made in the shape of a spiral of brass ribbon set in a slotted frame.
The dimensions of the helix are clearly apparent from the drawing. The primary is composed of seven turns of brass ribbon 1/2 inch wide and 1/16 inch thick. The secondary should have from 10 to 15 turns of ribbon 3/8 x 1/16. The coils may be slid back and forth on the brass rod so that the distance between them is variable. Connection is made to the coils by means of suitable clips. A clip similar to that shown on page 92, but made to snap on a flat ribbon instead of a round wire, will serve the purpose.
QUENCHED SPARK GAP.
A "quenched" gap is made up of a number of brass or copper disks accurately turned to a true surface and separated by mica or rubber rings about .01 inch thick. The spark discharge takes place in the air-tight space at the center of the disks, inside of the mica rings.
The quenched gap has several advantages over other forms. It is practically noiseless and the nuisance of a crashing discharge may be avoided by its use.
The large surface offered to the spark by the disks cools the spark and quickly stops the oscillations in the closed circuit, and thereby leaves the open circuit and aerial system free to vibrate in its own period and therefore radiates pure waves. By pure wave a wave of one length is meant.
A quenched gap cannot be used on a set of over 1 K.W. power without artificial cooling by an air blast.
Fig. 153. Quenched Gap.
Fig. 154. Quenched Gap.
Fig. 154 shows an efficient form of quenched gap for use in stations up to 1 K.W. in power.
The disks are shown in detail in Fig. 155. They are cast out of copper and then turned perfectly true and smooth in a lathe. After surfacing, the discharge surface should be heavily silver plated and buffed smooth.
Fig. 155. Details of Disk and Ring.
The disks are piled on a marble base with a mica ring between each. They are clamped down by a strong set screw mounted on a heavy brass yoke. Enough pressure should be brought to bear to force the plates tightly together and make them air tight.
Fig. 156. Explanatory Drawing of Quenched Gap.
The number of disks required is governed by the voltage of the charging condenser. Generally speaking it is one section of .01-inch gap for each thousand volts delivered by the secondary of the transformer. It is very important to secure just the proper number of disks. If properly adjusted, the quenched gap will give one discharge for each alternation of the current and produce a musical tone.
The quenched gap is placed in the same position in the transmitting circuit as any other form of gap.
ROTARY GAPS.
Rotary gaps are divided into two general classes, the synchronous gap and the non-synchronous gap.
The former usually consists of one or more stationary electrodes and a rotating member made like a star wheel with projecting spokes. This rotary member is attached directly to the shaft of the alternator or motor generator and arranged so that a spoke always comes opposite a stationary member at the exact moment that the maximum of potential is obtained in the condenser. Such an arrangement permits one discharge for each alternation of the current and produces a pure musical note easily distinguished in the telephone receivers at a distant station.
In the non-synchronous rotary gap the wheel is driven at a high rate of speed without any regard to synchronism with the alternations of the current.
The rotary gap shown in Fig. 157 is of the non-synchronous type.
Fig. 157. Amco Rotary Gap.
The rotating member is cast from an alloy of equal parts of zinc and aluminum. It is necessary to first make a wooden pattern from which the casting may be made. The details of the wheel are shown in Fig. 158. The casting must be placed in a lathe chuck and turned true. It is mounted on a hard rubber disk 2 7/8 inches in diameter and 1/4 of an inch thick. The disk serves to insulate the revolving electrodes from the motor shaft. The "rotor" is mounted upon the shaft by means of a small brass bushing which passes through the center of the disk.
Fig. 158. Details of Revolving Parts of Rotary Gap.
The motor must be well built and capable of running at high speed. A "Juno" motor will be found very satisfactory. When running free its speed is about 4500 r.p.m. With the rotor in place the speed is about 3600 r.p.m.
The motor should be mounted on a heavy marble base capable of absorbing any little vibration that the gap may be subject to when running at high speed.
The stationary electrodes are made in the same manner as those for an ordinary gap and consists of two flanged zinc electrodes mounted upon threaded brass rods supported by two hexagonal standards. The axis of the electrodes should be the same height above the base as that of the motor shaft.
The rotor should be carefully balanced so that it is practically free from vibration by boring small holes in the back face so as to make the weight on opposite sides equal.
Fig. 159. Details of Rotary Gap.
The motor may be driven by a battery or from the same source that supplies the transformer, in series with two or three suitable lamps. A motor wound to run directly from the 110-volt line or a higher potential must have its fields wound with very fine wire and is apt to give trouble through "burn-outs," due to "kick back." When the motor is operated on batteries or is wound for running in series with a lamp the danger is lessened.
A rotary gap is placed in the transmitting circuit in the same position as any other gap. Its use will result in a wonderful increase in the transmitting range of almost any station, for not only will the amount of energy passing through the aerial be raised, but the clear musical tone given off is more plainly distinguishable at a greater distance in the receiving station than a spark of the ordinary sort.
"KICK BACK."
The oscillations taking place in the closed circuit and aerial system of a wireless transmitter continue to surge after the current in the condenser has dropped below a certain value, and react upon the primary winding of the coil or transformer by induction and produce high voltage, high frequency currents termed "kick back," in wireless telegraph parlance.
"Kick back," wherever it exists to an appreciable extent, is liable to damage insulation and cause possible "burn-outs." The "kick-back" preventers illustrated in the accompanying diagrams will be found an efficient method of avoiding this danger.
Fig. 160. Methods of Preventing "Kick Back."
The first method shows an ordinary pressed telephone condenser of about two microfarads capacity connected directly across the A. C. mains near the transformer terminals, in series with two 6-ampere fuses. The condenser is shunted by a small spark gap made of needle points with a very small space, about .005 of an inch, between them.
The second method is an elaboration of the first and shows two sets of condensers in series with fuses and bridged by spark gaps with a "ground" through a third condenser connected between. This second method is the best and is often used to prevent delicate instruments, such as a voltmeter, from the effects of "kick back."
A proper "kick-back" preventer is part of the Fire Insurance Underwriters requirements for a wireless telegraph station.