682 Professor J. A. Fleming [May 24, 



periodic time of the oscillations in the antenna, and the wave length 

 as expressed by the formula wave length = velocity x periodic time. 

 Since the velocity is nearly 1000 million feet per second the wave 

 length in feet is easily found, when we know the time period of the 

 oscillations in the antenna. This last quantity can be found by 

 placing near to the antenna a circuit in which secondary electric 

 oscillations can be sympathetically induced by those in the antenna. 

 For this purpose we must have a circuit which possesses the two 

 qualities of capacity and inductance. This is secured by joining in 

 series some form of Leyden jar or condenser and some form of 

 spiral wire or inductance. Moreover, we must have the means of 

 varying this capacity and inductance, so as to bring the cymometer 

 circuit into tune, as it is called, with the antenna. Every such 

 circuit containing capacity and inductance has a natural period of 

 electric oscillation, resembling in this respect the time of swing of a 

 mechanical system composed of a heavy body suspended by an 

 elastic spring.* In my cymometer the condenser part consists of 

 one to four sliding tubes, each consisting of a pair of brass tubes, 

 separated by an ebonite tube. The outer tubes can slide off the 

 inner ones and so vary the capacity. The inductance consists of 

 a long spiral of copper wire, and the circuit is completed by a thick 

 copper bar. Matters are so arranged that when the outer tubes are 

 drawn off the inner tubes so as to vary the electrical capacity, the 

 effective amount of the spiral included in the circuit is simultaneously 

 varied in exactly the same proportion. To determine when the time 

 period of the cymometer circuit is in agreement with that of the 

 antenna, I use a neon vacuum tube. Some three years ago I found 

 that such a tube was extremely sensitive to a high frequency, electric 

 field being caused to glow brilliantly when subjected to its action. 



You are already familiar with the beautiful method discovered by 

 Sir James Dewar for obtaining neon from atmospheric air by the use 

 of charcoal at very low temperatures, and tubes filled with rarefied 

 neon prepared by his process are able, as I have shown, to serve im- 

 portant purposes in connection with wireless telegraphy. 



In the cymometer a neon tube is connected to the opposite 

 coatings of the condenser. If then the cymometer bar is placed 

 near to the lower part of a transmitting antenna, and we slide along 

 the outer condenser tube, thus varying the capacity and inductance 

 of the instrument, provided it has a suitable range, a position will be 

 found in which the neon tube glows brightly. The cymometer is 

 equipped with a scale which shows for every position of its handle 

 the corresponding frequency or time period, and the related wave 

 length. Hence the simplest opei'ation, which a child can perform, 

 serves to determine in one instant the frequency of the oscillations 



* If the capacity C is reckoned as usual in microfarads, and the inductance 

 L in centimetres, then the time period T of the oscillation is given by the 

 ormula T = V CL/5033000. 



