1905.] Measurement of ike Length of Long Electric, Waves, etc. 493 



set up in this condenser and wire by means of an induction coil as 

 usual, and the handle H is shifted until the vacuum tube V glows or 

 glows most brilliantly. We then know that the oscillation constant of 

 the instrument in that position agrees with that of the circuit so 

 formed. When that is the case, the oscillation constant of the wave- 

 meter can be read off in the scale attached to it, and we, therefore, 

 know the oscillation constant (call it 0) of the circuit formed by the 

 condenser and the wire. Hence, if C is the capacity of the condenser 

 in that circuit and L is the inductance of the wire of the circuit, then 

 O = ,yCL. If, then, we increase the inductance L by adding in series 

 with it a wire of which the inductance is desired (call it L'), we can 



FIG. 4. 



H 



then shift the handle H until we get a fresh agreement and find a 

 second value 0' for the oscillation constant of the circuit. Then we 

 have 



0' 



Hence L' 



As an illustration the following test measurement was made. A 

 copper wire, the diameter (d) of which was 0-128 or 032 cm. was bent 

 into a nearly complete circle 70 cms. in diameter. The inductance ( 

 this wire can be calculated from the formula 



i; 



-2-45 



27T71 



In the above case D = 70 cms., and Z = 0-33 cm., and B is he 

 resistance of the wire to oscillations having a frequency n. Tt 

 of L' is then 2400 cms. This wire was joined in series with another 

 wire laid alongside the har L* L 4 of the measuring instrument and a 

 ^d "serhavfng a capacity of 0'OOH6 microfarad joined up in series 

 with the circuit and a spark gap. The instrument was then used to 

 determine the oscillation constant 0' of the circuit with the circular 



