Oscar F. Dame
There are two types of condensers which date back to the earliest days of electricity, before Franklin's time, when the only known electrical generators were the statistical machine and the electrophorus. These two types were the Leyden jar and the glass plate substitute for the Leyden jar.
The Leyden jar was used in connection with cylinder and plate static machines to gather the electricity collected by the "brushes" and hold the charge to the maximum capacity of the jar, when the discharge would take place disruptively and with great volume of sound. These jars were made of thin clear glass, similar to the ones in use today in all laboratories, with both inside and outside coated with tinfoil to one-half or two-thirds the entire height. The capacity of each jar in micro-farads was infinitely small, the average being roughly estimated at about .0005 M. F.
Consideringcapacity in micro-farads, as we speak of it in connection with induction coil and telephone and telegraph practices of today, it would necessitate thousands of such jars to equal in capacity the modern type of small condensers which can be slipped into one's pocket with ease. There is a scientfic reason for this, which should be understood by the amateur worker, because of the value of the information in wireless telegraphy as well as in other pursuits. Three things enter into the calculation of capacity: - The area of the thin metals we utilize for the purpose; the distance we separate these surfaces one from the other, that is to say, how far apart they are separated either in dry air, in oil or by means of an insulating substance like glass, mica, hard rubber, paraffined paper or the like; and thirdly, the value of this intervening substance as a dielectric or insulator.
We might take the best of metals, such as thin lead or tin-foil, and separate the sheets with poor, unwaxed paper, and get a very faulty condenser. We might use poor mica, full of pin-holes or flaws, or hard sheet rubber with metal specks in it, all of which would in time render the condenser worthless. In the selection of the insulating material we have two standards, oil and mica, the former the best liquid medium, and the latter the best solid. For oils, we have kerosene, paraf-fine oil, transel oil, linseed oil, etc.; for mica we have all grades at all prices, the best grades being very costly. Mica is not obtainable in very large pieces, the average commercial sizes seldom being over 6 in. square. Consequently mica condensers must be for very small capacities or built up of many layers. All the highest priced testing apparatus used in laboratory work have mica condensers, and many of them are silvered similar to mirrors instead of being coated with thin metal sheeting. For general purposes glass would answer, but is not procurable in sufficiently thin pieces.
As was before mentioned, the distance between opposite faces regulates the capacity. The sheets of metal of certain size, separated 1-64 in. apart, give just twice the capacity when separated 1-128 in. Then, again, if the material used in separating them was paraffine paper of ordinary quality, we could not expect so great a capacity, to use a shellac coating between, which in some cases improves the capacity coefficient. The co-efficient of inductive capacity is not as much considered in modern practice as it should be. Nowadays many foreign made induction coils are equipped with thin, poorly waxed paper dielectrics, which will hardly stand the strain of a primary core discharge of not over 20 volts.
This weakness demonstrates itself in the tiny thread like sparks from the secondary; the charge which should have returned to the primary winding short-circuiting itself in the condenser dielectric. High-grade condensers for coil work are made in two ways In those of foreign make it is the usual custom to cut the foil and paper in small squares and connect the metal sheets alternately to common terminals, that is to say, 1, 3, 5, 7, and so on, form one terminal, and 2, 4, 6 and 8 the other. In ordinary condenser work in this country it is customary to arrange the condenser materials in long, narrow strips, the paper and foils being on rolls in such position that two sheets of very thin bond paper with a strip of foil between, will pass through a sort of clothes wringer attachment, through paraffine wax, and emerge stuck together as one. Such a strip forms a completely insulated side for a condenser, and two of these sheets about 5 in. wide and several yards long, may be laid one on the other and folded or rolled into desired shape.
There is one disadvantage about such a condenser, however, which has been overlooked by most American coil makers, and that is the "unloading value" of a condenser. Disregarding all that has been written about condenser theories, every man knows, whether electrician or not, that the more of the tail-board of a wagon taken out, the quicker the load may be dumped. Now, when a condenser is made of two long, insulated strips of foil, flattened or rolled into shape, such a condenser is sluggish. It does not un. load quickly. The charge stored within it leaks out through a sort of resistance which dissipates the quantity of electricity the condenser was calculated to hold and release. The reason these condensers are made this way is because of simplicity and cheapness of construction. Competition requires a minimum of expense and labor in all departments of coil manufacture, and in this type of construction there is a saving of fully 20 minutes work on each coil.
The latest type of condenser, however, is made up of two strips of the thin bond paper, as before described, and the roll of tinfoil between, but the foil instead of being narrow and hardly reaching within one inch of the paper's edge for the sake of insulation, projects on one side a full inch. In rolling up such a condenser, two of these strips are placed so that the foil of No. 1 projects on the opposite side from that of the No. 2. The sheets are then rolled in flat form and the foil projecting from the first side crimped together for one terminal of the condenser, and the other side pressed or crimped for the other terminal. Such a condenser will "unload" with a rush. When used on gas-engine coils having vibrators going at the rate of 1200 a minute, this condenser demonstrates its peculiar fitness for the work by giving a very lively and flaming secondary spark. Even on wireless telegraph coils the spark value is increased one-quarter by its use.