Harold P. Davis
The amateur telegrapher working on a line connecting the residences of one or two friends, has undoubt-ly wished many times that he had some device which would enable him to repeat the "call" for several minutes at a time and so avoid being obliged to do so by hand. Repeating the same letters from ten to thirty minutes at a time becomes both monotonous and tiresome, and when the operator at the other station is busy with other matter this is sometimes necessary.
The repeater which is here described provides a way for calling an office mechanically, and will run long •enough with one winding of the weight to "raise" the desired office if the operator there visits the room at all frequently. It is easily made from materials to be found in about every household, or any parts lacking can be obtained with but little trouble. The requirements are: The movement from an old clock, a double and single pulley as used for awnings, a clean tin can as used for coffee, some strip brass and braes rods and wood for a base and posts, as will be described.
The can should have one inch of depth for each " call " to be made, and an equal space for the belt. Measure off the can and cut to the required depth with tinner's snips, using care to avoid dents in the cutting. The cover is then put on and lightly soldered in place. Locate the exact center of each end and punch holes for the shaft, which may be cut from a piece of brass rod, as used for sash curtains. The shaft is cut to a length to project 1/2 in. at one end and 1 in.
at the other. The shaft is then soldered in place.
The wooden posts H are nailed to the sides of the base, which should be wide enough to allow the posts to carry the shaft with small collars between the ends of the can and the posts. These collars may be made of small spools, such as twist spools. A strip of spring brass T, about 1/2 in. wide is fastened to the base with a round brass screw, or a binding post may be used, which would be more convenient in making connections. The upper end of the strip is curved outwardly, and the strip is located to press lightly but evenly upon the end of the can, so as to make good contact.
The other contact, which is movable, is made from a piece of spring brass wire of about l-16th in. diameter. A piece about seven or eight inches long will be required, as several turns must be made near the center to form a bearing for the rod upon which it slides. On the end which rests upon the can must be soldered a short piece of strip brass like that used for the first contact. A piece of brass rod like that used for the shaft and a little longer than the round surface of the can must be supported at the ends by small blocks in which are bored holes only part way through. The sliding contact is put on the rod, and the rod placed in position by fastening the blocks to the base with screws. About two inches away from the rod is fastened a strip of wood, in the upper edges of which are cut slots spaced the same distance apart as the cali stencils on the can. The end of the sliding contact rests in the proper slot to sound the desired call.
The stencils for the different calls are made from thick, strong manilla paper, which is cut into strips about 1 in. wide and long enough to go around the can and have a small lap at the joint. The speed at which the can will turn is first ascertained, and the letters of the call are then cut out, or rather spaces corresponding to the dots and dashes making up the letters. The strip is then coated on both sides with shellac and immediately placed in position around the can. It should be smoothed down so as to present an even surface to the contact as it revolves. Any shellac which may run out into the open spaces should be removed, so that the can will make a good electrical contact with the sliding arm. As many of these paper stencils are made as there are stations to call. After all the stencils are in place the outside surfaces can again be coated with shellac, which with the paper makes a good insulation where the arm is not to touch the can.
To rotate the can, part of the train of gears from an old alarm clock will be needed. Movements of this kind can generally be had for the asking from many jewelry stores. The part of the train to be retained begins with the mainspring shaft and the three shafts and gears directly connected to it. The mainspring shaft projects through the side for an inch or more, and on this projecting end should be secured as large a wooden spool as can be made to hold on the shaft. One way of fastening to the shaft is to file a flat place on the shaft and put a screw through the spool until it reaches the flat space, thus acting as a set screw. On the shaft at the other end of the train a smaller wooden cylinder is fitted and fastened by means of two or three small screws, the heads of which bear on the gear on that shaft. This cylinder can be split for convenience in fitting, and the two parts united by glue, which will hold if reinforced by several turns of fine wire, as the duty is not heavy. From this pulley is run a belt made of tape which also runs over one end of the stencil can. The clock movement must be fastened to the base so that the tape belt will run in line.
To move the gears, and in turn the stencil can, a cord is attached to the spool on the mainspring shaft, the end of which is carried through the hole in the base and a weight attached. The weight is rigged just the same as in old-time clocks, by means of the awning pulleys, one being attached to the weight and one to the under side of the base. The quickest way tore-wind the cord on the spool after it is all run out is to pull in the cord and wind it around the spool by hand.
In connecting this calling device into the circuit, it may be wired in shunt with the regular key, and cut out by means of a switch when not wanted. In this way it can be located in any convenient place in the room. It seems quite probable that this device could be used for wireless telegraphy if made a little heavier so as to take the larger current, but never having tried it for that work, cannot state absolutely that it would be satisfactory when used in that way.