CHAPTER VII. OSCILLATION CONDENSERS AND LEYDEN JARS.
A condenser consists of two conducting surfaces separated by an insulator or dielectric. Fig. 53 shows a diagram of a simple condenser in which A and B are two tinfoil sheets separated by a sheet of glass, C.
Fig. 53. Simple Condenser.
If A is connected by means of a wire to a static machine a positive charge will collect on the glass at A and induce a negative charge at B, so that if A and B are connected to a small spark gap the charge will leap the gap in the form of a spark.
When a condenser discharges through a coil of wire, the discharge consists of a large number of exceedingly rapid oscillations or surgings. The first passage of current more than empties the condenser and it becomes charged in the opposite direction, that is, the conducting coatings change their polarity. A reverse discharge then occurs which also oversteps itself and the oscillations thus go on but become rapidly weaker until they die completely. The time consumed in the discharge may have been only a fraction of a second, but during that short period the current perhaps oscillated several thousand times.
If a condenser is discharged through a conductor of high resistance the discharge passes out slowly, and dies away gradually in one direction without oscillating. One of the fundamental equations of wireless telegraphy is therefore that there will be oscillations in a circuit if the resistance in ohms is not greater than the square root of four times the inductance in henries divided by the capacity of the condenser in microfarads.
The capacity or the ability of a condenser to store electricity depends upon the area and form of the conducting surfaces, the thickness of the dielectric between them, and a factor known as the specific inductive capacity of the dielectric. The unit of capacity is called the farad and is defined as the condenser which would be raised to a potential of one volt by a charge of one ampere flowing for one second. A condenser of such a capacity is, because of its enormous size, impractical to construct, and the unit ordinarily used is therefore the microfarad, or one millionth of a farad.
Capacity may be calculated from the following formula:
Capacity equals K(A/D),
where K equals a constant depending upon the specific inductive capacity of the dielectric, A the total area of tinfoil and D the thickness of the dielectric.
Leyden Jars.—Transmitting condensers in a wireless telegraph station usually take the convenient form of a jar, coated inside and out with tinfoil and known as a Leyden jar.
The jars should be of good Bohemian or Jena hard glass and coated with tinfoil only for about three-quarters of their height, as otherwise the discharge is liable to pass over the top. The tinfoil must be thick to avoid blistering, and is stuck to the glass with shellac varnish. The blistering of Leyden jars is a serious fault, for when this condition exists, the capacity is thereby altered to such an extent that the period of the closed circuit may be sufficiently altered to throw the system out of tune and decrease the radiation of energy.
Considerable expense may be saved if the glass jars are purchased and coated by the amateur. The best jars are those imported from Germany, which have wide mouths so that they may be easily coated inside with tinfoil.
Fig. 54. Leyden Jar.
The jars must be thoroughly cleaned and dried before they are coated. Give the inside a thorough brushing over with shellac varnish, and before it is dry, carefully insert the tinfoil and press it smoothly against the glass. The outside of the jar is treated and coated in the same manner. The inside and outside of the bottom are also coated by cutting the tinfoil in circular pieces and shellacking them on.
Fig. 55. "Aerial Switch."
The whole upper part of the jar is given one or two coats of shellac in order to prevent the collection of moisture and brush discharging. A wooden plug fitted in the top of the jar supports a brass rod, terminating at the lower end in a chain or spiral spring which connects with the inner coating. When trouble is experienced because of an imperfect contact between the coating and the chain or rod, a layer of brass filings an inch or two deep placed in the bottom of the jar will remedy the difficulty. The upper end of the rod usually terminates in a small brass ball or a binding post.
The wooden plug or cover is dried in an oven to expel all moisture and then boiled in paraffin.
Small Leyden jars may be very conveniently made from six-inch test tubes and mounted in a rack so that the capacity of the condenser will be adjustable by removing one or more of the tubes. An ordinary test tube rack such as is used in chemical laboratories serves very well for this purpose. The tubes should be connected in parallel, that is, all the outside coatings together and all the inside coatings together.
Figs. 55 and 56 illustrate condensers of this type which are on the market. The tubes are all separately removable so that the capacity may be adjusted.
Fig. 56. Amco Oscillation Condenser.
Glass Plate Condensers.—Glass plate condensers offer several advantages over Leyden jars and are coming into wide use. They are not so bulky or expensive and, above all, do not blister.
Plate condensers are often placed in a rack and made adjustable by means of movable contacts. Much the better plan is to place the plates in oil, as this eliminates all corona or brush discharges and much sharper tuning is rendered possible. The container is usually a tight wooden box filled with oil or paraffin after the plates are in place.
It is impossible to state the size of condenser suitable for induction coils of a given power or spark length, because many factors such as inductance, length of aerial, etc., which differ in various stations, influence the capacity. A condenser of convenient size suitable for coils or small transformers consuming from 250 to 300 watts is that described below. It is about the proper size for the small open core and 1/4-K.W. closed core transformers, described in the last chapter.
Fig. 57. Clapp-Eastham Oscillation Condenser.
The glass plates may be secured by removing the emulsion from old 8 x 10 inch photograph plates. Hot water will soften the gelatin on the plates so that it may be very easily scraped off. Twenty-four plates of this size are required. The tinfoil is cut 8 x 8 inches, so that an inch margin is left on all sides. The alternate sheets are connected together by heavy tinfoil or thin copper foil strips. The condenser should be placed in a convenient sized wooden box and poured full of paraffin.
The plate condenser shown in Fig. 57 is of .02 microfarad capacity. The condenser is mounted in a plain wooden box with several binding posts brought out, so that the capacity may be varied by connecting in various sections. The condenser is manufactured and designed by the Clapp-Eastham Company for use with the transformer illustrated in Fig. 51.
It is very necessary to have the transmitting condenser adjustable so that its capacity may be varied, for the proper value depends upon the wave length, spark frequency, power and persistency of the wave train.
When the condenser capacity is too small the spark will be somewhat flaming like an arc, and the potential to which the aerial is charged will be low. If too much capacity is used the spark will be very irregular and intermittent.
Fig. 58. Methods of Varying Capacity.
Fig. 58 shows condensers connected in series and in parallel and a combination of the two. Two condensers of equal capacity connected in parallel have twice the capacity of one, while in series they will have only one-half the capacity of either. This may be otherwise stated as the capacity in series is equal to the reciprocal of the sum of the reciprocals of their capacities separately. By this means of connecting either in series or in parallel almost any desired adjustment of capacity may be brought about.
Oftentimes a high voltage may be divided between two condensers by placing them in series and thus using them safely on a voltage which would rupture either one alone. For example, two condensers built for 20,000 volts and to be used on 30,000 volts could be made to perform this duty safely and only undergo a potential of 15,000 volts, which is a large margin of safety.
It is obvious that if the capacity of the circuit were to remain constant, four condensers connected up in series-parallel would be necessary.
In case of several Ley den jars or condensers connected up in a transmitting circuit, the leads or conducting wires connecting the various units should all be of as nearly the same length as it is possible to have them.
The resistance of metallic conductors to high frequency currents is several times their normal resistance to constant currents. The larger the diameter of the wire the greater is this ratio. This increase of resistance is due to the fact that the high frequency currents permeate wires only a very short distance. In the case of copper, the depth is only about one three-hundredth of an inch and with other metals much less. Therefore it is advisable to use as leads and conductors of large condensers, stranded wires or flat ribbons of sheet copper in order to present more surface and offer less resistance than solid conductors of an equal cross sectional area.
Iron must never be used, as its resistance to these currents is over fifty times that of copper.
After connections are once established between the jars or the condenser units, they cannot be altered nor the capacity changed without re tuning the circuits afterwards.