Oscar N. Dame
In constructing a receiving outfit, the same aerial and ground is used, and provision is made by a jack-knife switch by which the aerial may be shifted from receiving to sending at will. A synchronizing coil similar to the one described in the October number is also constructed, with three connecting clips for attaching portions of apparatus to the coils. The lower end of this coil is permanently grounded, and the upper end is connected to one side of the coherer. The other side of the coherer goes to the coherer battery, whence continues the circuit through the windings of the relay back to the lower end of the coil where the ground connection is made.
For the protection of the delicate apparatus in use, there is placed in parallel across this circuit, a lightning spark gap, designed to carry foreign discharges direct to ground, should they enter on the aerial. The sounder and decoherer are connected to the relay, as in the sketch here shown.
In selecting a pole for the aerial wire, the locality in which one lives plays an important part. In many sections of the country it is an easy matter to procure straight poles with eight inch butts and four inch tops, 60 feet long, or longer, but in other States the best that can be procured is the shorter Cananian spruce poles, averaging 30 feet in length. These may be procured in all diameters in nearly all cities in the North and East for about S3 each at the yards, and smaller poles for the topmast for about $2. In this way a 50 foot pole may be had for $5, including such small iron work as a blacksmith would provide for stepping the topmast. Such a pole sould be set in the ground fully five feet and braced a few inches below the earth's surface by logs or timberbutts placed crossways about the pole and bolted together. This bracing adds greatly to the endurance of the pole in stormy weather.
The question might be asked: "With a certain height of pole, how far may one receive and send?" To this it is difficult to reply with any degree of accuracy, a great deal depending on the apparatus in use. It is absolutely certain that such a pole will suffice for picking up messages from commercial stations located even twenty miles away, and will, with a 2-inch coil _ transmit across a township and possibly 10 miles, pro vided atmospheric conditions were favorable. But it must be fully appreciated that sensitive receiving devices are necessary for this distance, and it is the writer's opinion that no home constructed receiving device will be found available for such a distance as 10 miles unless constructed with great skill and accuracy, while on the other hand, the ordinary glass tubs coherer may be found sufficiently accurate up to 3 or 4 miles.
There are places in this country where the purchase of a Canada tree is out or the question, and in such places recourse may generally be had to saw-mill and lumber yards. Spruce sticks 6x6 in. and 30 ft. long make a very creditable mast if tapered on four sides by means of the rip saw at the mill. A top-mast of the spruce 4 x 4 in. and likewise tapered is easily prepared, but both mast and topmast should be made of selected wood free from knots and flaws, for a cut timber is inferior to a natural stick for flagpole purposes owing to its lack of flexibility.
In connecting topmast to mast it is desirable to provide means of lowering the mast during stormy weath" er, especially if the stick is of small diameter. It is customary to have a ring permanently fastened to the mast at the top, through which the topmast is drawn by means of halyards, the but of the topmast finally being set in position on a pin, as shown in Fig. 2. With ordinary poles of less than 55 ft. height, this may be dispensed with, as the topmast may be easily lifted into place by one man with the aid of a ladder.
Aerial wires should terminate in an insulated knob of glass so as to be readily attached to and drawn up by the halyards. This permits raising and lowering a •cage or other device designed to improve the receiving and sending efficiency.
In designing a cage for experimental work it is best to have the cage length just one-fourth the extreme length of the aerial wire. The main wire should be No, 14 or No. 16 bare copper. Iron wire may be used where copper wire is obtainable only at considerable cost.
In bringing aerial wire into the instrument room, it is advisable to provide a better insulation than would be had with a porcelain tube inserted in the wall. The writer has seen a small round hole drilled in the glass pane of the window, for the entering wire. A convenient way to arrange this would be to construct a wooden framework sash of sufficient height to take 4 x 5 or 5 x 7 glass plates (old photo negatives will prove very useful for this purpose), and a hole drilled in one pane for the aerial wire. Should it be desirable to bring the ground wire in through this frame, the second hole should be drilled as far as possible from the first, to insure insulation. As the length of the aerial is measured from the coil to the peak of the wave cage it is desirable to have the wire run direct to the instrument room without any bends and with as few loops as possible. The wire should not be permitted to rest on trees or any part of the building, or on the pole by which it is raised.
A small brass or galvanized iron pulley is fastened by a staple or serew eye to the top, for the halyards. The aerial wire may consist of any of the various forms of cages or fans calculated to assist in sending or re" ceiving, or may be a single wire fitted with a glass or porcelain insulator to which the halyards are made fast.