558 POPULAR SCIENCE MONTHLY. 



length and having a resistance of about thirty ohms. This little 

 loop is sealed into a glass bulb like a very small incandescent lamp, 

 or it may be enclosed in a small silver bulb and the air may be 

 exhausted. If an electrical oscillation is sent through this exceed- 

 ingly fine platinum wire, it heats it and rapidly increases its re- 

 sistance. The electrical oscillations produced in an aerial are sent 

 through a number of these loops arranged in parallel, and the loops 

 are short-circuited by a telephone, joined in series with a source of 

 very small electromotive force produced by shunting a single cell or 

 opposing to one another two cells of nearly equal electromotive force. 

 Any variation of resistance of the little j)latinum loops due to the heat 

 produced by the oscillations, by suddenly altering the current flowing 

 through the telephone, will cause a sound to be heard in it. The elec- 

 trical oscillations when passing through the loops are therefore de- 

 tected by the heat which they generate in these exquisitely fine plati- 

 num wires. 



Finally, one word must be said on the subject of electrodynamic 

 receivers, due to the same inventor. An exceedingly small silver ring 

 is suspended by a quartz fiber and has a mirror attached to it in the 

 manner of a galvanometer. This ring is suspended between two coils 

 joined in series, which are placed either in the circuit of the aerial or 

 in the secondary circuit of the small air core transformer inserted 

 between the aerial and the earth. When electrical oscillations travel 

 down the aerial they induce other electrical oscillations in the silver 

 ring, and if the ring is so placed that its normal position is with its 

 plane inclined at an angle of forty-five degrees to the place of the 

 fixed coils, then the ring will be slightly deflected every time an 

 oscillation occurs in the aerial. 



Omitting further mention of the details of the kumascopes in use 

 and the receiving aerial, we must next proceed to consider the receiving 

 arrangements taken as a whole. 



In the original Marconi system, the sensitive tube or coherer was 

 inserted between the bottom of the receiving aerial and the earth.* 

 Accordingly, when the incident electric wave strikes the receiving 

 aerial and creates in it an oscillatory electromotive force, this last will, 

 if of sufficient' amplitude, cause the particles of the coherer to cohere 

 and become conductive. This sudden change from a nearly perfect 

 non-conductivity to a conductive condition is made to act as a switch 

 or relay, closing or completing the circuit of a single cell, and so send- 

 ing a current through an ordinary telegraphic relay, closing or com- 

 pleting the circuit of a single cell, which may in turn actuate another 

 recording telegraphic instrument, such as a Morse printer. To pre- 

 vent the oscillations from passing into the relay circuit, small choking 

 * See British Patent Specification, G. Marconi, No. 12,039, June 2, 1896. 



