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.

When the apparatus is in operation D C S is closed, so that the current from B operates the coherer (D C). Directly communication is broken off, the switch (D C S) should be opened; otherwise the buzzer would keep up a continuous tapping. For long-distance work a more efficient sending apparatus is shown in Fig. 35. This is composed of an induction-coil, with the terminal rods and brass balls forming the spark-gap, an oil key (K), and three or more large storage-cells, or a dynamo (if power can be had to run it). A condenser is placed in connection with the aerial and ground wires, so that added intensity or higher voltage is given the spark as it leaps across the gap. In operation this apparatus is similar to the one already described. Where contact is made with K the primary coil is charged, and by induction the current affects the secondary coil, the current or high voltage from which is stored in the condenser. When a sufficient quantity is accumulated the spark leaps across S G and affects wires A W and G W. This action is almost instantaneous, and directly the impulse sets the ether in motion the same impulse is recorded on the distant coherers and sounders.

There are a great many modifications of this apparatus, but the principles are practically the same, and while the construction of this apparatus is within the ability of the average boy, many of the more complicated forms of coherers and other parts would be beyond his knowledge and skill. Marconi has realized his ambition to send messages across the ocean without wires, and is now doing so on a commercial basis, and at the rate of twenty-five words a minute. It is but the next step to establish communication half-way around the world, and finally to girdle the earth.