DRY AND WET NON-SPARKING SWITCHES
A base of wood three by five inches is made and given several coats of shellac. Obtain a small rubber or composition pill or salve box, and make it fast to the front end of the base with an oval-headed brass screw driven down through the centre of the box. A wire leading to one binding-post is arranged to come into contact with the screw, and the other post is connected by wire to one hinge-plate supporting the bar. The long machine screw, or rivet, passed down through the knob and into the bar, extends down below the bar for half an inch or more, so that when the knob is pressed down the end of the screw, or rivet, will strike the top of the screw at the bottom of the box without the bar coming in contact with the edge of the box. When in operation the composition box is filled with olive oil or thin machinery oil, so that when contact is made by pressing the knob down the circuit will be instantly broken, the spring at the rear end of the bar drawing it back to rest. The oil prevents any sparks jumping across; and also breaks an arc, should one form between the contact-points. With the addition of a good storage-battery (the strength of which must be governed by the size of the induction-coil and the distance the messages are sent) and a dry-cell or two for the receiving apparatus, the parts of the wireless apparatus are now ready for assembling. Full directions for making storage-cells is given in [chapter ii.], [page 21], and for dry-cells in chapter ii., [page 29]. For short-distance work the plan shown in [Figs. 33] and [34] will be found a very satisfactory form of apparatus. One of each kind of instrument should be at every point where communication is to be established.
In the sending apparatus ([Fig. 33]) S C are the storage-cells, K the key, and I C the induction-coil. T T are the terminals and balls, S G the spark-gap, and P P the posts that hold the terminal rods. A W is the aerial wire running up from one post, and G W the ground-wire connecting the other terminal post with the ground-plates.
In the receiving apparatus ([Fig. 34]) C is the coherer, D C the de-coherer, T S the telegraphic sounder, or relay, and A G the astatic galvanometer. B is the dry-cell, or battery, and D C S the de-coherer switch, so that when the apparatus is not in use the dry-cell will not operate the buzzer or de-coherer. A W is the aerial wire and G W the ground-wire. Two or more storage-cells may be connected in series (that is, the negative of one with the positive pole of the other) until a sufficiently powerful source of current is secured for the transmission of messages.
To operate the apparatus, the circuit is closed with K, and the current from S C flows around the primary coil in I C and affects the secondary coil, causing the spark to leap across the gap (S G). This causes a disturbance through the wires A W and G W, and the ether waves are set in oscillatory motion from the antennæ on the house-top. This affects the antennæ at the receiving-point, and the impression is recorded through the coherer (C) on the telegraphic sounder or relay (T S), which is operated by the current from dry-cell or battery (B), since the oscillations have broken the resistance of the filings in the coherer (C). The instant that the current passes through the coherer and operates T S, the astatic galvanometer indicates the presence of current by the deflected needle.
