Velocity of Electric Waves in Air. 1-19 



The platinum globules were first brought into the slightest 

 possible contact by carefully adjusting the micrometer-screw. 

 When such a contact has been effected was known by watching 

 the motion of the milliamperemeter-needle. Upon causing 

 a train of electric waves to pass the coherer, the resistance of 

 the latter was lessened by the globules moving into closer 

 contact, that is to say, the globules were made to cohere. 

 This augmentation of the cohering of the globules was instantly 

 seen by the increased throw of the milliamperemeter-needle. 

 The coherer employed was the outcome of a great many ex- 

 periments with Lodge coherers, Marconi coherers, or Branly 

 tubes. Tubes containing metal filings of all kinds and of 

 different degrees of fineness, such as iron, silver, platinum, 

 copper, zinc, nickel, magnesium, brass, granulated arc-carbon 

 with copper, or brass, or iron terminals, either magnetized or 

 non magnetized, all proved to be unsuited for the measuring 

 of the electric wave-lengths. They all had the common fault 

 of not allowing the milliamperemeter-needle to return quickly 

 to its zero reading. Decohering of the coherer in the present 

 form is accomplished almost instantly by the elasticity of the 

 platinum spirals upon opening the circuit-key. No tapping- 

 is required as in other kinds of coherers. Moreover, the 

 above coherer at once shows the manner of action of the 

 ordinarv m eta 1-fi lings coherer. The platinum globules are 

 brought into slightest contact, the passage of an electric wave 

 causes the globules to cohere more closely; thus the resistance 

 to the battery-current through the coherer is lessened, as is 

 shown by the increased deflexion of the needle. The cohering 

 of the metal filings (in this case the two globules) was probably 

 due to an electrostatic effect produced upon them by the 

 passage of the electromagnetic waves. Further evidence of 

 this will be given later. 



Before concluding the description of the coherer, it will be 

 necessary to give an account of the oscillator-circuit and its 

 accessories. 



The electric spark which sent forth the trains of electro- 

 magnetic waves through the free air between the oscillator 

 and coherer, was produced by a large Ruhmkorff coil in 

 connexion with a storage-battery of 27 cells in parallel with 

 a voltage of about 55. 



The oscillator consisted of two platinum globules, 1 mm. in 

 diameter, attached to the ends of a platinum spiral making- 

 two convolutions about copper terminals, exactly like those in 

 the coherer. This oscillator was placed in the secondary 

 spark-gap of the Ruhmkorff coil. The oscillator globules 

 were always kept 4 mm. apart from each other. The same 



