HERTZIAN WAVJ'J WIRELESS TELEGRAPHY. 555 



jacent wave trains are sent, these will indicate themselves by producing 

 a rapid series of ticks in the telephone, heard as a short continuous 

 noise and taken as equivalent to the Morse da^h. 



It was by means of this remarkably ingenious instrument that Mr. 

 Marconi was able, in the summer of 1902, to detect the waves sent out 

 from Poldhu on the coast of Cornwall, and receive messages as far as 

 Cronstadt in the Baltic, in one direction, and as far as Spezzia in the 

 Mediterranean in another direction, and also to receive messages across 

 the Atlantic from the power stations situated in Glace Bay, ISTova 

 Scotia, and from one at Caj)e Cod in Massachusetts, U. S. A., in 

 December, 1902. 



There can be no question that this magnetic detector of Mr. Mar- 

 coni's, used in connection with a good telephone and an acute human 

 ear, is the most sensitive device yet invented for the detection of elec- 

 tric waves and their utilization in telegraphy without continuous wires. 

 It is marvelously simple, ingenious and yet effective, as a Hertzian 

 wave telegraphic receiver. 



Whilst on the subject of magnetic wave detectors, the author may 

 describe experiments that he has been recently making to construct a 

 Hertzian wave detector on the Eutherford principle, wliich shall be 

 strictly quantitative. All the receivers of the coherer type and electro- 

 lytic type give no indications that are at all proportional to the energy 

 of the incident wave. Their indications are more or less accidental and 

 depend upon the manner in which the receiver was last left. There is 

 a great need for a quantitative wave detector, the indications of which 

 shall give us a measure of the energy of the arriving wave. It is only 

 by the possession of such an instrument that we can hope to study 

 properly the sending powers of various transmitters or the efficiency of 

 different forms of aerial or devices by which the wave is produced. 

 This magnetic receiver is constructed as follows : 



A coil of fine wire is constructed in sections like the secondary 

 coil of an induction coil, and in the instrument already made, this 

 coil contains thirty or forty thousand turns of wire. In the interior of 

 this coil are placed a number of little bundles of fine iron wire wound 

 round with two coils, a fine wire coil which is a magnetizing 

 coil, and a thicker wire coil which is a demagnetizing coil. These 

 sets of coils are joined up, respectively, in series or in parallel. 

 Then, associated with this form of induction coil is a commutator of 

 a peculiar kind, which performs the following functions when a bat- 

 tery is connected to it and when it is made to revolve by a motor or 

 by clockwork. First, during part of the revolution, the commutator 

 closes the battery circuit and magnetizes the iron cores, and whilst this 

 is taking place the secondary circuit of the induction coil is short- 

 circuited and the galvanometer is disconnected from it. Secondly, the 



