Page:Popular Science Monthly Volume 92.djvu/655

 Popular ISciencc Motdhly

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��over the surface of the plate. The spark passes between the rounded end of the brass tube and the flat surface, and hot conducting gases formed by the discharge are blown away by the current of air. By using large masses of metal and a strong blower, fairly heavy oscillating cur- rents may be passed across the gap with- out overheating — especially if a low group frequency is used so that the gap may cool off somewhat between succes- sive sparks. A gap of this sort was used in the Eiffel Tower station which sent signals across the Atlantic to Arlington, Virginia, in the international longitude experiments of some years ago. It is even more successful with smaller pow- ers, and represents, perhaps, the best practice in the so-called stationary open spark-gaps.

In the next article, the construction and action of the rotary and modern "quenched" gaps will be described, and thereafter the interactions of the two oscillating circuits and the production of radio waves will be discussed.

{To he continued)

��A Simple Compact Short Distance Wireless Telephone

VARIOUS methods have been devised by which one can talk without wires to a person over four thousand iniles, but it is not generally known that in 1882 Prof. Alexander Graham Bell constructed an apparatus with which he was able to transmit the voice several miles without metallic conductors. The principle in- volved was that of magnetic induction, which, simply explained, is a field of force induced by passing an electric current through a coil of wire. This so-called "field of force" is again detected by a similar coil placed in a direct line with the source. The limited distance that the voice could be transmitted, due to me- chanical difficulties, prohibited such an apparatus from being of commercial value, but for experimental purposes, or for com- municating short distances, such as be- tween rooms or nearby buildings, it is practical and it is something unusual for the amateur to build.

The accompanying drawings illustrate

��a compact outfit suitable for talking to some one 50 to 100 ft. away.

A base-board A, somewhat like the one shown, should be screwed to the wall at a convenient height for talking. A common coat-hook is fastened to the peak.

Then make a large coil of insulated bell-wire (about 3 or 4 ft. in diameter) using about 350 ft. of wire. Number 20 gage is a good size for all practical pur- poses, but the finer the wire and the

���One of the two apparatus used for sending messages by home-made wireless telephone

greater number of turns with a cor- responding increase in the diameter of the coil, the greater is the distance one can talk. Leave 1 ft. or more of the ends of wire projecting and wrap the entire coil securely with tire-tape. The coil is then ready to ?iang over the hook on A and should be large enough to hang clear of the bottom edge of A.

A two-way knife switch B of the style shown, is attached to the base of A and the center terminals are connected to the ends of the coil C and D. One side of the switch E is then connected to a pony tele- phone receiver F, which can be hung from a convenient hook near a telephone transmitter G. This transmitter is con- nected to the opposite poles of the switch H as shown; a dry-cell / cutting in one line. The battery is fastened to A by means of a pair of metal straps.

A duplicate apparatus should be placed at the other end of the "line," in a j>osi- tion as nearly parallel as possible, to re- ceive the full benefit of the "field of

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