Contacts.
It is absolutely essential that the diameter of contacts for all contact breakers should be as large as possible and their faces filed truly parallel to enable them to easily carry all the current required. One of the main causes of failure of coil is burning of the platinum point and platinum burr, the current being then materially reduced. Large sparks at point of rupture are often indications that the condenser is not working properly—perhaps has broken down or is not large enough. The contacts will sometimes fuse together; at any rate, the excessive sparking is an evidence of waste as much as in a dynamo generator.
The adjustable method of arranging condensers (see Chapter IV.) is here of great value, but it is easy to attach more condenser sections to the contact screw pillar and vibrator pillar and notice result. In the construction of Ruhmkorff coils it is a good plan to make all connections possible on the coil base, instead of inside the condenser chamber. This is done either by means of rubber-covered wires or neat strips of brass, screwed down on the base from points of connection, and, of course, carefully bent over or well insulated from all other leads which they have to cross.
The best makers of induction coils construct their instruments so that they can be readily taken apart with as little detachment of connections as possible.
CHAPTER III.
INSULATIONS AND CEMENTS.
In selecting an insulating compound for apparatus designed to be under the influence of high tension currents, a glance at some of the peculiarities of such currents will not be out of place. Mineral oil is used in many of the converters employed to transform the high voltage currents on the mains of the alternating electric-light systems to the comparatively low voltage used at the points of consumption. Professor Elihu Thomson, in a series of experiments, noticed some interesting facts in the sparking distances of high potentials in oils.
He found that discharges of low frequencies, as 125 alternations per second, were capable of puncturing mineral oils at one third to one half the thickness of an air layer sufficient to just resist punctures by the same discharge; but with frequencies of 50,000 to 100,000 per second, an oil thickness of one thirtieth to one sixtieth was a sufficient barrier.
At a frequency of 125 per second, a half-inch spark in the air penetrated one third to one fourth inch of oil; but at frequencies of 50,000 to 100,000 per second, a layer of oil one fourth of an inch successfully resisted the passage of a spark which freely passed through 8 inches of air.
The effect of drying an oil improved its insulating qualities. (Tesla uses boiled-out linseed-oil.)
He also noted that pointed electrodes could be brought nearer together under oil than balls without allowing a discharge. Flat plates allowed of still greater sparking distances. Tesla notes that oil through which sparks have passed must be discarded, probably owing to particles of carbon being formed.
Paraffin wax has a higher resistance than oil, providing it has not been heated over 135° C. It will stand alternate heating up to 100° C. and cooling, being of lower resistance when hot than when cold. But a serious permanent deterioration takes place when it has been heated over 100°C.; its color, from the normal pure white, changes to a yellowish tint when its insulation is impaired. Paraffin also undergoes a deterioration when heated for a long time even at 100° C., and should never be used for fine work when it is at all yellow. It is always best to melt it in a hot-water bath, not permitting, however, any steam or moisture to come near it. In this climate (United States) it is not so necessary to mix in any tallow to obviate brittleness, the average temperature of most workshops being sufficiently high to keep it from becoming brittle.
Resin oils do not suffer permanent injury from being heated, as does paraffin, but their insulating properties diminish much more rapidly on becoming even warm, the initial resistance of resin oils being lower than that of paraffin.
Paraffin has a fault—its tendency to absorb a slight degree of moisture. It has been found in telephone and telegraph cables saturated with paraffin that this is a very important cause of their deterioration. In Ruhmkorff coils, however, which are intended for operation in enclosed places free from damp atmospheres, the absorption of moisture would be probably reduced to its minimum.
There is one substance which, were it not for its cost, would be far preferable to paraffin for coil work, and that is beeswax. Its cost, however, is generally five times that of paraffin, even when purchased in quantities. It never becomes brittle enough to be damaged in careful handling, its melting point is low, and it does not absorb moisture. But it must be unquestionably pure and clear.
In foreign practice a variety of resinous mixtures are used to insulate the turns of the wire in Ruhmkorff coils.
Equal parts of resin and beeswax used hot, paraffin, resin and tallow, and shellac and resin are employed.
Shellac—that is, the yellow lac—is much used as a varnish for electrical instruments, being dissolved in alcohol to saturation. For dynamo armatures and similar apparatus the shellac varnish is of great service, and many good compounds of shellac, such as insullac and armalac, have been prepared for ready use. But (excluding beeswax) for our purposes paraffin stands pre-eminently at the head of the list.
In using shellac varnish, in high tension work more particularly, care must be taken that the moisture has entirely evaporated. Although a piece of shellacked apparatus may appear perfectly dry, yet when the current is allowed to flow unlooked-for results may appear—it takes hours in a dry atmosphere for shellac varnish to dry. Baking the apparatus in a warm oven is a necessary expedient whenever feasible, care being taken not to burn or decompose the shellac. The proportions most generally used are 1 ounce shellac to 5 ounces alcohol. Stand the vessel containing the mixture in a warm place, and shake it frequently; filtration improves the varnish somewhat.
A ready and efficient varnish for silk is prepared by mixing 6 ounces of boiled linseed-oil and 2 ounces of rectified spirits of turpentine. For paper, 1 part of Canada balsam and 2 parts of spirits of turpentine dissolved in a warm place and filtered before being used. A good insulating cement for Leyden jars and insulating stands is prepared from sulphur, 100 parts; tallow, 2 parts, and resin, 2 parts, melted together until of the consistence of syrup, and sufficient powdered glass added to make a paste. To be heated when applied, this will resist most acids. The resin and beeswax compound is handy when making experimental mercurial air pumps of glass tubes, as it has a fair tenacity, is not too brittle, and is easily used.
CHAPTER IV.
CONDENSERS.
A condenser is an apparatus whereby a charge of electrical energy may be temporarily stored, the amount of energy it will hold determining its "capacity." The capacity of a condenser is measured in micro-farads, the commercial unit representing one millionth of a farad. A farad equals the capacity of a body raised to the potential of one volt by a charge of one ampere for one second at one volt—i.e. = one coulomb.
The measurement of the capacity of a condenser is accomplished by the use of a ballistic galvanometer. The latter instrument has a bell-shaped magnet suspended in a coil of fine wire. When a momentary current is passed through this coil the magnet hardly commences to rotate until the current has practically ceased. A beam of light is reflected from a mirror fixed to the magnet on to a scale. The degree of deflection is compared with that obtained by the discharge of a condenser of known capacity, and the capacity of the condenser being measured is deduced by a simple rule. The farad, which is the unit of capacity requiring a condenser of an immense size, is replaced by a commercial unit, the micro-farad—that is, one millionth of a farad.
The original form of the condenser was the Leyden jar, which owes its name from the town of Leyden in Europe.
Fig. 29.
The Leyden jar is made as follows (Fig. 29): A clean uncracked glass jar with a wide mouth is coated on the inside and outside with tinfoil; sometimes loose tinfoil is filled inside, the tinfoil, however, not reaching more than two thirds of the jar's length from the bottom. A cork is fitted, and through the middle of it a wire is passed touching the inside coating of tinfoil and terminating in a metal sphere outside. A simple Leyden jar can be made in a few moments by half filling a glass bottle with water and wetting the lower half of the outside; a wire run through the cork into the water finishes the job. But this is at least only a makeshift, although a fair amount of current has been collected from a leather engine belt in motion in one thus made.
A condenser can be easily made as follows (Fig. 30):
Fig. 30.
Procure a clear glass plate, G, free from flaws, 11 inches square by 3/33∕32 inch thick. Give this a good coating of shellac varnish all over, sides and edges. Cut out of smooth tinfoil two sheets, T, 8 inches square, and round off the corners with a pair of shears. There must be no sharp corners, projections, or angles to induce leakage. Lay the glass plate on a sheet of paper, and mark its outline thereon with a pencil; then remove it and substitute a sheet of the tinfoil, and mark that. This will enable you to centre the foil. Give one side of the glass plate another coat of varnish, and so lay it on the paper that its outline coincides with the pencil outline. When the varnish has partly dried take a sheet of the trimmed foil, and by observing the pencilled marks you can lay it on the varnished plate exactly in the centre. Lay down the top edge first along this line, and carefully deposit the remainder of the foil in place. Next, with a flat brush full of varnish go over the plate, pressing out any air bubbles, and ensuring both a flat and a well-varnished surface. When this is dry, turn over the plate and repeat the operation on the other side.
If desired, a metal hemisphere of at least an inch in diameter may be attached with varnish, first scraping the foil to make a contact. The whole plate can be swung in a cradle of two silk threads, laid on a glass tumbler, or mounted on end in a shellacked block of wood.
A strip of tinfoil, S, attached at the corner can be used as a connector. The plates must be joined in the following manner when two or more are used in conjunction, and a quantity of current is desired. They should be placed so the connecting strips project alternately from each side (Fig. 31), and all on each side joined so as to leave two terminals, one to the 1, 3, 5 plates, the other to the 2, 4, 6 plates, and so on, which, when joined, will have the same effect as would result from the use of two large plates of the same total area. The nearer the plates are together the greater capacity they will have, always supposing the insulation is good, the insulation being known as the dielectric. Another good method, when a high quality of glass can be procured, is to lay the tinfoil on the plates without varnish, piling one on top of the other, tinfoil and glass alternately, and clamping the whole securely, laying a piece of cloth top and bottom to avoid cracking the glass from the pressure. This must be kept from moisture; a strip of paraffined paper stuck along the edges and extra paraffin run on will answer very well.
Fig. 31.
In constructing these glass condensers, they must be designed to correspond with the coil with which they are to be charged. In the foregoing description we have allowed a margin of 1½ inches of glass around the foil coatings. This will make 3 inches as the maximum distance between the coatings. Although a 2-inch spark from the coil would not jump this interval, a certain discharge will take place, and the less this occurs, the more serviceable the condenser will be. Therefore a greater margin should be allowed for a longer spark than 2 inches.
In the commercial condenser for telephone and telegraph use, paraffin and paper are substituted for glass, as will be described later. Heavy paraffin oil gives excellent results, but its fluidity is disadvantageous.
There is no valid reason why paraffin could not be used on the glass plate condensers, care being observed that it is free from dirt and metallic chips. In fact, the space between the glass plates of the multiplate condenser may be filled in with paraffin, and thereby exclude the air. Only a condenser so built up is not convenient to take apart for experimental purposes.
The foregoing description of a glass insulated condenser was written with the assumption that a good quality of glass be used. But the ordinary window glass is generally useless, and paraffined paper is preferable. The quality of glass known as "hard flint glass" is best, the superior qualities being imported from Europe. This latter is used in the manufacture of the standard Leyden jar for lecture purposes.
Were it not for its cost, the finest dielectric we could use would be sheet mica. Unfortunately sheet mica over 3 inches square is expensive, and becomes rapidly more so as it becomes larger.
Standard condensers for testing are made with mica carefully selected, and retain the charge for the maximum length of time. The built-up mica condenser is immersed in molten paraffin until the same has permeated the sheets, and then the complete mass is put under a pressure until the paraffin is well set.