Owing to the long time required to complete a condenser to the working condition, it is described at this time rather than after the rest of the work on the coil is finished. The particular trouble referred to is the difficulty of drying out all moisture; this being necessary to guard against the possibility of sparking through the insulation and consequent failure of the condenser. One condenser made by the writer proved absolutely worthless because of the presence of moisture in the shellac, which was used as the dielectric. My experience has been that carefully waxed thin bond paper makes the best dielectric for amateur's use, two thicknesses of paper being placed between the sheets of foil.

Assuming, for the sake of illustration, that the condenser to be made is to be used on a spark coil rated to give a four inch spark, we find that the condenser requires approximately 3000 square inches of foil. It is necessary to state that condenser capacity is a variable quantity; no two condensers made from the same specifications having exactly the same capacity, but the results will be near enough to the estimated capacity so that no difficulty will arise from minor divergencies.

As the various uses to which a coil is put require different condenser capacities, it is desirable to make up the condenser in four sections with a plug or other large contact connecting device, which will permit of one or more sections being used as needed. This gives to each section 750 square inches, which works out nicely for sheets of foil 5 x 6 in. =25 sheets, each containing 30 square inches. To make an even division and also allow for loss or corners, 28 sheets will be used, 14 for each end, it being understood that the alternate sheets are connected as will be described. For the four sections there will be needed about 4- pounds of tin foil in sheets 5 in. wide, 1 ream of legal size thin bond paper and about 3 pounds of pure paraffine wax.

The legal size paper measures 8 x 14 in. and should be taken to a printer's and cut to 8x7 in. At the same time one diagnonally opposite corner should be cut off to a mitre cut of 45°. Measure off 3 in. each way from the corners and cut to the points thus obtained. The foil is then cut to the size 5 x6 in. and one corner cut off, measuring 2 1/2in. from the corner for the points for the mitre.

To make up the condenser, an agateware pan 9 x 12 n. will be needed; also an iron pan about 10 x 14 in.

and a squegee roller. The agateware pan is to contain the paraffine wax, and is placed in the iron pan; the space between the two being filled with water and the wax heated by means of the water. To heat the wax directly on a stove involves the risk of setting it on fire, which is avoided by using the water bath. If an oil stove is available it is preferable to use it rather than work over a cook stove, as considerable time is required and the work is rather "messy." A squegee well suited for the purpose can be made from a piece of glass tubing with thick walls, plugging the ends with wood and fitting a handle thereto. The ends of the tube should be ground smooth on a grindstone.

Induction Coil Making For Amateurs III The Condens 168

The materials being placed conveniently at hand, melt the wax in the agateware pan. When melted to quite a fluid condition, the wax is sufficiently transparent so that the foil and paper can be seen at the bottom. The wax should be boiled for at least half an hour before proceeding further. The paper should also be dried for several hours in an oven moderately heated, using care not to scorch the paper.

When all is in readiness, place two sheets of paper on the bottom of the pan, one at a time, and roll with the squegee roller. Then place a sheet of foil in the center of the paper with the corner overlapping the cut in the paper. Add two more sheets of paper, then another sheet of foil with the corner projecting at the end opposite that of the first sheet. Two more sheets of paper are followed by another sheet of foil; this time the corner projects at the same end as does the first sheet. The process is continued until all the sheets of paper and foil are laid for a section, the top layer being two other sheets of the paper. It will be seen that all the even numbered sheets of foil project at one end and the odd numbered sheets at the other.

After laying each sheet of foil on paper, roll smooth with the squegee to force out all air bubbles. An even, firm pressure, rather than a heavy one, is what is needed. The efficiency of the condenser is largely dependent upon the success with which the air is excluded, so the work should not be hurried.

After all the sheets are laid, the section should remain in the boiling wax for an hour or more. It is then removed and placed between two pieces of thin, strong wood, or thick binders board, which have previously been well dried in an oven and also soaked in the wax, binding all solidly together with lineman's adhesive tape, except at the corners where tin foil projects.

To clean the wax from the projecting corners of the foil suspend in turpentine until the wax is all dissolved, and then clean off the turpentine by dipping in grain alcohol. The projecting corners of foil are then firmly pressed together, using care not to break them from the sheets of which they are a part. In the center is placed a strip of thin brass l 1/2x1/2 in. having a i in. hole at about the center of the projecting corner. Punch a hole through all the corners of foil and place therein a binding post having a screw of small wire gauge, and screw the nut down tight. Copper or brass washers should be placed under both the head and the nut of the binding post to prevent tearing the foil. To the projecting brass strips are soldered the connecting wires, which can be done much more easily and safely than by attempting to solder the wires directly to the foil, as the latter melts easily and repairing is then a difficult matter.