552 POPULAR SCIENCE MONTHLY. 



Marchese Luigi Solari, of the Eoyal Italian Navy. One of the plugs 

 of this tube is connected to the aerial and the other to the earth, and 

 they are also connected through another circuit composed of a single 

 dry cell and a telephone. The arrangement then forms an extremely 

 sensitive detector of electric waves or of small electromotive forces, or, 

 if a wave falls on the aerial, the electromotive force at once improves 

 the contact between the mercury and the plugs and therefore causes 

 a sudden increase in the current through the telephone, giving rise to 

 a sound; but when the wave ceases, or the electromotive force is with- 

 drawn, the resistance falls back again to its origin value, and the 

 arrangement is therefore self-acting, requiring no tapping or other 

 device for restoring it to receptivity. 



A very ingenious form of combined telephone and coherer has been 

 devised by T. Tommasina.* In this instrument the diaphragm of an 

 ordinary Bell telephone carries upon it a very small carbon or metallic 

 coherer. This coherer is connected in between the aerial and the earth, 

 and is also in circuit with a battery and the electromagnet of a tele- 

 graphic relay. When this relay operates it closes the circuit of another 

 battery which is placed in series with the telephone coil. The moment 

 the current passes through the telephone coil it attracts, and therefore 

 vibrates, the diaphragm and shakes up the metallic filings. If an 

 observer therefore places the telephone to his ear, he hears a sound cor- 

 responding to every train of waves incident upon the aerial. With this 

 arrangement, one can obtain two different kinds of results, according 

 to the nature of the cohering powder placed in the cavity in the 

 diaphragm. First, if the powder consists of a non-magnetic metal, 

 gold, silver, platinum or the like, the receiver will be very sensitive : 

 and at the same time the current passing through it when it is colierod 

 will be sufficient to work a sensitive recording apparatus in series with 

 the telephone coil. Secondly, if the metallic powder placed in the 

 cavity is a magnetic metal, the receiver will be somewhat less sensitive, 

 but will work with more precision, because of the magnetic action of 

 the magnet of the telephone upon the cohering powder. If no record- 

 ing apparatus is used, the observer must write down the signals as 

 heard in the telephone, since corresponding to a short spark at the 

 transmitting station, a single tick or short sound is heard at the tele- 

 phone, and corresponding to a series of rapidly successive sparks, a 

 more prolonged sound is heard in the telephone. These two sounds, as 

 already explained, constitute the dot and the dash of the Morse signals. 



We may, in the next place, refer to that form of kumascope in 



which the action of the wave or of electromotive force is not to decrease 



the resistance of a contact, l)iit to increase that of an iin])erfect contact. 



As already mentioned, Professor Branly discovered long ago that 



* See U. S. A. Patent Specification, No. 700,101, May 24, 1900. 



