Popular ISciencc Motdhly 



G39 



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 



