Velocity of Electric Waves in Air. 121 



needle readings will vary so irregularly as to be absolutely 

 worthless. Sparks of the same nature must therefore be 

 produced at all times during the experiment. This is rendered 

 possible by the employment of the continuous stream of water 

 over the surface of the mercury in the mercury-cup of the 

 break. 



The coherer and its capacity are mounted upon, a stand 

 carried upon a carriage which can be easily moved along a 

 graduated track. It is possible to make a change in position 

 of the carriage as small as "25 mm. To the side of the 

 carriage is firmly secured a shelf which supports the keys of 

 the two circuits, the shunt-resistance, and the milliampere- 

 meter. The lower part of the carriage holds the battery and 

 its resistance-box. 



If the coherer be placed in any part of the room, it re- 

 sponds to the sparking of the oscillator, but in some positions 

 more strongly than in others. It thus appeared desirable to 

 place the oscillator within a completely-closed metal box, 

 with a window in its front side. This proved beneficial, 

 since it caused the waves to be less scattered at the instant 

 of leaving the oscillator, confining them more to that region 

 of the room where the experiment is conducted than else- 

 where. The metal box then acts, as it were, like a mega- 

 phone. The front of the box was on hinges, and could be 

 left open at pleasure. The inside of the metal box was put 

 into metal connexion with the gas-pipes of the room. At 

 one end of the room was placed the oscillator, while at the 

 opposite end was fixed a metal reflector, which was connected 

 by wires to gas- pipes and water-pipes. In the free space 

 between the oscillator and reflector moved the carriage- 

 coherer. The dimensions of free space between the reflector 

 and oscillator were 12*67 m. in length, 6 - 15 m. in width, and 

 5*14 m. in height. The nearest wall had five windows, each 

 1*66 m. by 1*12 m. The reflector was of sheet-tin, 3*32 m. 

 wide by 4*24 m. high. 



The oscillator, the coherer-globules, and the centre of the 

 reflector were always kept in the same horizontal line. The 

 oscillator and coherer v/ere thus 2*25 m. above the floor, 

 2*89 m. below the ceiling, and 1'57 m. from the nearest wall, 

 which ran the full length of the room. By moving the 

 carriage to and fro the coherer was always in the same 

 horizontal line at any desired distance from the reflector. 



The coherer-carriage was moved to any distance from the 

 reflector, its globules being put into slightest contact ; then 

 the coherer circuit was closed by touching the coherer-circuit 

 key; and the milliamperemeter-needle reading taken. Upon 



