This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
Oscar F. Dame
During a recent visit to a large wireless telegraph station, I observed in use a special type of condenser of very simple construction, and when immersed in insulating oil, so highly efficient that immediately on my return I set about making one for my own laboratory, where it now fills a long felt want.
In wireless work the services of an adjustable condenser are invaluable, but most adjustable condensers are simply banks or sets of small condensers which may be connected in series or multiple to give a few selected capacities. With an electrolytic receiver the condenser must be adjustable by every gradual and minute steps, otherwise the sounds heard in the head have four plates electrically connected, but l/8tb of an inch apart.
With a pair of dividers two circles are scribed on tin plate, 8 in. in diameter, and then each circle divided in two equal parts. With a pair of tinsmith's shears, the four half-discs are cut out evenly, care being taken to not bend or warp the discs, for each disc is to slip in between the square iron sheets previously mentioned, without touching at any part. At the point which was used as a center in outlining the circles, or exactly 4 in. in from the edge, it is advisable to punch or cut a 3/16 in. hole before cutting out the discs. Then, when the cutting is completed, we have a metal piece telephone will be faint and uncertain. At one time a bulky type of adjustable condenser was made from two telescopic tubes of glass lined with tin-foil, the inner tube being pushed more or less into the other tube to give desired capacity. Such a decice was bungling and very liable to breakage and finally gave way to the type here described.
In constructing the condenser, first purchase some extra heavy stove tin in perfectly flat sheets. From two of these cut four pieces 10 in. long by 5 1/2 in. wide. In the four corners of these pieces holes are punched to take four 8/32 machine screws. The four screws are thrust through the holes in one piece of plate, and then iron washers of a thickness of 1/8 in. slipped onto the screws. Then another plate is placed, and another set of washers, and so on until the four plates are in place, when the nuts are turned down securely onto the screws. By this arrangement we shaped as in Fig. 1.
A longer 8/32 machine screw than those previously used is next procured, also a supply of nuts to fit same. These nuts should be as near 1/8 in. thick as possible. The discs are next mounted on this machine screw, by screwing a nut securely against each disc until 4 discs are firmly fixed in place, one above the other, and 1/8 of an inch apart. The with soder the discs, nuts and screws are firmly affixed together.
A base-board of 1/2 in. pine or white-wood 12 ins. square is next procured and carefully shellaced on both sides and the edges. Holes are bored to take the four machine screws used in constructing the square plates, and the four screws slipped through and fastened with a final nut on each. This brings the bottom plate close to the wooden base. Calculations must now be made to find the proper position of the half circular pieces in their relation to the fixed plates, for as will probably be surmised by this time, the machine screw supporting them is to form a pivot or staff by which the half-discs are to revolve, thereby permitting the discs or vanes to intermesh as much or as little as desired, and without touching the permanent iron sheets at any point. This latter is imperative.
Having ascertained the exact position of this machine screw pivot, the head of the machine screw is cut off, and the exact diameter of the part un-cut with threads taken with calipers. With a drill of this size, a hole is bored in a block of brass or iron which is used to support the vanes erect. A hard-rubber knob is affixed to the other end by means of which the discs may be swung back and forth as desired without danger of discharging the electrcity through the body.
Having completed these metal parts, and connected two binding posts, one to the pivoting block, and the other to the fixed vanes, and having assured ourselves by actual test that the vanes swing clear of each other, we will now proceed to construct a wooden case for the condenser, which will contain parafine oil, when finished, as an insulator. This box will measure 12 in. square inside, and of a hight 1/2 in. above the highest vane. If the maker feels certain that he can construct a case with corners tight enough to hold parafine oil. a cover may also be built of proper size and a small indicating needle affixed to the pivot screw outside the cover, by which the exact location of the moving vanes may be ascertained without removing the cover. One may experience difficulty in constructing a case to hold oil, and I would advise that a second case 1 in. larger all around be built, and the first one sealed in place with plaster o fparis, which will furnish a positively secure container for the oil.
This condenser is by far the best I possess for universal use. I find it invaluable with an electrolytic receiver, and have also substituted it for Leyden jars in connection with the secondary discharge of an induction coil. The only chance for a break down is where, through over-sight, the moving discs are permitted to touch or come too close to the fixed plates.