TKAX^ACTIOXS OF SKCTIOX A. 



547 



G. Mafjni'tic Dfitectors in Space Teleyrcqyhy.^ 

 By Professor Ernest Wilson. 



This 13 a subject which has attracted considerable attention since Mr. Marconi 

 has found that as a substitute for the coherer magnetic detectors give ver}' con- 

 sistent results over long- distances. Kutherford was probably the first to make a 

 scientific investio-ation into the effect which high frequency electric currents have 

 upon the magnetism of a bundle of wires previously magnetised. ^ The author of 

 the paper also made experiments with magnetic detectors in 1807.^ One form of 

 his detector consists of a ring of fine iron wires the magnetism of which can be 

 continuously reversed Ijy a local battery. A telephone connected to a coil wound 

 on the ring is used to receive the minute transient electric currents produced wlien 

 another coil on the ring receives the electric impulses from a distant transmitter. 

 Using straif^ht iron wires on which the coils are wound the author finds that tlin 

 effects are increased by submitting the specimen to pull and twist stress, and iiho 

 by heating it to a temperature a little below that at which iron ceases to be a 

 magnet isable substance. 



7. A New Receiver for Hertzian Oscillations. 

 By Professor G. M. Minchin, F.R.S. 



About three years ago I constructed a receiver (' coherer ') for electromagnet'c 

 oscillations, in which the effective bodies are carbon and aluminium. The descrip- 

 tion is as follows : — C is a cylinder of electric-light carbon about 

 meter, and about the same length, terminating in two spindles ; 

 A.\A is an aluminium wire, about ^^y inch in diameter, bent 

 round at its ends into two circular arcs into which the narrow 

 spindles of the carbon cylinder tit somewhat loosely. The 

 cylinder C is attached to a platinum wire/), and tlie alumi- 

 nium wire to a platinum wire P. The system j^)CAAA is 

 inserted into a glass tube containing mercury at its closed 

 end, and into tliis mercury the wire p dips. The wire P 

 parses through a narrow neck at the other end of the tube. 

 The mercury is boiled so as to expel the air from the tube, 

 and when the air is expelled the narrow neck is closed and 

 P sealed into the tube. Another platinum wire Q was 

 initially sealed into the lower end, so as to make contact with 

 the mercury. 



Thus the system of carbon cylinder and alumininum wire 

 is now sealed into and suspended inside a glass tube contain- 

 ing mercury and its vapour. 



Tried in the laboratory with the oscillations produced by 

 an electrical gas lighter, this proved to be a very sen.^itive 

 receiver. 



It was simply placed as a resistance in the circuit of a 

 weak dry cell and a telephone, and the oscillations of the gas 

 lighter affected it at the distance of about 30 or 40 feet. 



Xo tapper is necessary to break the coherence. 



When I made tliis receiver and wished to try it over long 

 distances my induction coil most unfortunately broke down; 

 and, for one reason or another, I was iioable to try it until 

 the beginning of August 1902. 



The oscillator consisted of a rectangular sheet of zinc, about -3 feet by 6 feet, 

 elevated to the top of a metallic telegraph post about 25 feet high. The rectiving 

 surface was a sheet of copper, of about half the area of the zinc, at'ached tea 

 wooden pole about 12 feet high at a distance of about 600 yards from the oscillator, 



' See the Electrician, September 2C, 1902. 



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