10 Chant — Experimental Investigation into the 



ber of vibrations. For most of the work the key was depressed 

 for 10 swings of the pendulum, that is, for about 60 interrup- 

 tions of the coil. Thus in the primary circuit the current 

 traversed in succession the primary of the induction coil, the 

 motor-break, the pendulum rod into the mercury and the 

 circuit-key. The battery consisted of 10 storage cells arranged 

 in two banks in multiple, i. e., giving 10 volts of E.M.F. 



The parabolic mirrors, both for oscillator and receiver, were 

 75 cms high, 60 cms wide, and had a focal length of 12'5 cms . At 

 first reflectors of but 5 cms focal length were tried, but they 

 were entirely useless, the reflected waves apparently destroy- 

 ing completely the direct waves.* 



3. The Receiver. 



As a receiver I used the magnetic detector devised by 

 Butherfordf and described by him in 1896. A few inches of 

 iron wire 0'14 mm in diameter were dipped in melted paraffine 

 and, after cooling, the wire was cut into about 20 pieces. 

 Holding these together, about 90 turns of fine (No. 40) insu- 

 lated copper wire were wound upon about l cm of their length ; 

 and after the protruding ends of the iron wire had been cut 

 oft' and the fine wire had been soldered to heavier copper 

 wires, the helix with its iron core was mounted in the end of 

 a small glass tube, the connecting wires being drawn through 

 the other end of the glass tube. The helix was kept in place 

 by simply dipping the tube in melted paraffine and then allow- 

 ing it to cool. The entire arrangement is shown (half size) 

 in fig. 6. The helix is seen at H, and the ends of the wire 

 are shown at <?, e, curved as they were used. 



On a wooden rod about 65 cms long two wires (fig. 7) were 

 fastened with soft wax, their near-together ends terminating 

 in small mercury cups. The bar was cut out at its central 

 portion so that the detector D could hang vertically with its 

 ends in the mercury cups. Thus the wings became electrically 

 a portion of the wire of the helix. The wooden bar was 

 mounted in a parabolic reflector so that the wire lay along a 

 focal line (fig. 5). 



The method of using was as follows : The iron of the 

 detector was first magnetized to saturation by being thrust 

 into a helix in which a current was made to circulate. The 

 detector was then hung so as to connect with its wings, and on 

 placing it in the field of an electric oscillator, the surgings 

 gathered up by the wings destroyed a portion of the mag- 

 netization of the iron. The change in the magnetization was 

 observed in a magnetometer. 



* I had forgotten Hertz's experience. See Electric Waves, pp. 112, 175. 

 | Phil. Trans., A 1897, vol. 189, pp. 1-24. 



