Warren S. Preston
In wireless telegraph experiments there is occasional need of a variable resistance to be introduced in the different circuits, especially in the coherer and relay circuit, where it is necessary to balance the amount of current flowing through the coherer so that the relay will not be energized until the filing structure in the coherer chamber is broken down by the in-coming wave. In building such a piece of apparatus it is advisable to provide for other uses besides wireless telegraphy, so that the outfit may be of general use in the laboratory.
The first detail of construction is a box-like base, the top for the switches and sliding resistance, and the inside for the fixed resistance coils. This is made of mahogany or other well seasoned stock, size 10x12 in. and 1/2 in. thick. The sides should be 2 in. deep. A bottom is not necessary, but may be provided. The genera) design of this base is given in the illustration.
In winding resistance coils, it is customary to wind two wires at one time joining the two wires atone end, thereby making a non-inductive circuit. In a small resistance it has been the writer's experience that this provision is hardly necessary, and in the lower resistances of this set this theory will be disregarded.
It is a very simple matter to estimate from an authentic table of wire resistance the approximate number of inches of any gauge of wire necessary to give a desired resistance, and the finer the wire used for the purpose, the greater the latitude in cutting to measure without affecting the resistance. In these days of improved wire-drawing machinery, wire of all gauges is drawn exactly true and even, but it is not feasible to figure entirely by these gauge tables, because no two manufacturers use the same metallic formula for their wire stock, consequently 36 inches of one maker's No. 36 wire might measure one whole ohm more resistance than another's.
It will therefore be necessary to provide for this variance by using one brand of No. 36 single cotton covered wire for the whole instrument. The American wire gauge tables give the resistance of this gauge as 414 ohms per thousand feet or 2.4217 feet to the ohm.
As few amateurs are provided with instruments at home for measuring 2.48 feet of this wire to this scale, the amateur should visit or correspond with some school, college or private laboratory in regard to this, giving them an unstretched sample of the wire to test. In about 20 different brands of No. 36 magnet wire tested by the writer, very little variance was noted in the length of wire giving one ohm, but of course in 100 feet a decided difference was noted.
One interesting detail in this connection, is the relation of wire gauge to resistance. For example, a wire 3 sizes larger than another will have twice the sectional area and one-half the resistance for equal lengths. Reference to any table will prove this. A No. 36 wire contains 25 circular mills sectional area and measures 414 ohms to the 1000 feet while No. 33 is 50 circular mills and measures 206.99 ohms to the 1000 feet.
Our first resistance coil will be one ohm, and one of the simplest forms to wind upon is the ordinary thread spool. The exact length should be found by measuring with an accurate resistance test set, but 2.42 feet will prove very close. This wire should be wound neatly upon the spool with two inches free at each end for soldering connections. The next resistance will be 10 ohms and will consist of 24.22 feet of wire. The third resistance will be 25 ohms and consist of 60.55 feet of wire. The fourth resistance will be 160 ohms and consist of 242.17 feet wire.
These 4 coils are mounted in the base box with wood screws placed in the holes in the spools. Two binding posts are placed at one side of the base board, and a connection clip and connection cord attached to each, as in sketch. " Five pins or heavy brads are driven through the base board a half inch or so, leaving about one-quarter of an inch exposed at the top. To the bottorn of each pin is soldered wires from each coil in this fashion: Commencing at the right, the outer end of the 1 ohm coil is soldered to one brad. The other end is joined with the first end on the 10 ohm coil and both soldered to pin or brad No. 2. The other end of the 10 ohm coil is joined to the first end of the 26 ohm coil and both joined to the brad No. 3, etc., etc. The last terminal is soldered to a final brad. It will be seen that by means of the clip arrangement any one or all of these coils may be introduced in a circuit by just clipping on to the brads. The combined resistance is 136 ohms.
Provision may now be made for a single wire resistance of No. 36 bare German silver wire, the resistance of which per foot must be ascertained from the manufacturer, as this wire is drawn to all percentages and hardly any two brands will measure the same. Thirty feet of this wire is fastened by one end to a pin driven into the base board and is then tightly strung back and forth across the base board around wooden pegs made of 1/2 in. dowelling set into the base board, as in sketch. The extreme end of the wire is soldered to another flexible cord and clip, with thit wire resistance the exact resistance of which per inch has been ascertained, and the coils, any combination of resistances may be connected up to balance the coherer circuit, thereby adding greatly to the efficiency of the apparatus.