1905.] Measurement of the Length of Long Electric Waves, etc. 491 



The pins p lt p 2 (see fig. 1) which form, respectively, one of the 

 terminations of the inductance and capacity, are connected together by 

 a stout copper bar LI, L 2 , L 3 , L 4 , so completing the electric circuit, the 

 capacity and inductance of which can be varied. In order to detect 

 the condition in which the oscillations have a maximum value in this 

 circuit, a vacuum tube V is employed (see fig. 1), which may preferably 

 be a vacuum tube containing neon, or failing that, a tube containing 

 rarified carbonic dioxide may be used, the glass being uranium. This 

 vacuum tube should be constructed with two bulbs and with a narrow 

 tubular portion like a spectroscope tube. It may be attached to the 

 outer brass jacket as in fig. 1, or it may be attached to a bar connected 

 with the inner brass tube, the vacuum tube being hung over the outer 

 jacket. Also a scale is provided showing the position of the sliding 

 jacket and which, therefore, can be graduated to show the oscillation 

 constant of the arrangement for various positions of the jacket. 



Supposing then that we desire to determine the frequency of the 

 oscillations in any wire such as a Marconi aerial wire used in Hertzian- 

 wave telegraphy, part of this wire is laid alongside the copper bar and 

 the oscillations in it induce others in the circuit of the wave-meter. The 

 oscillation constant is then varied by moving the outer jacket by means 

 of the insulated handle H until the vacuum tube V glows most brightly. 

 If proper adjustments are made of the position of the vacuum tube, it 

 will be found that the tube does not shine at all until the outer jacket 

 J is within a few millimetres of the position in which the oscillation 

 constant of the instrument agrees with that of the circuit being tested. 

 By taking two or three readings with the jacket, a little too far one 

 way and a little too far the other way and approaching the right 

 position from both sides and taking the mean scale reading, it is 

 possible to obtain the oscillation constant with great accuracy. If the 

 capacity is measured in microfarads and the inductance in centimetres, 

 then it will be found that the oscillation constant required is generally 

 some number lying between 1 and 20. The instrument already con- 

 structed by the author on this pattern is adapted for the determination 

 of oscillation constants lying between 0'16 and 7 '5. 



In the case of Hertzian-wave telegraphy conducted on Marconi 

 principles, the wave-length employed, reckoned in feet, is equal to 

 195'8 times the oscillation constant of the transmitter circuit, or 

 reckoned in metres, to 59 '73 times the oscillation constant. Hence, 

 the above described instrument is capable of measuring wave-lengths 

 from 30 to 1500 feet, and might easily be constructed to measure 

 wave-lengths of any greater length.* 



By a slight modification, the instrument can be constructed more 

 simply as follows: A single ebonite tube is employed which may be 



* February 11, 1905. The author has since constructed one to measure electric 

 waves up to 2400 feet in length. 



