[ 758 ] 



LXXIT. The Application of the Cymometer to the Determination 

 of the Coefficient of Coupling of Oscillation Transformers. 

 By J. A. Fleming, M.A., D.Sc, F.R.S., Professor of 

 Electrical Engineering in University College, London*. 



[Plate XI.] 



THE author lias already described one form of direct- 

 reading cymometer for the determination of wave- 

 lengths and frequencies in connexion with electric-wave 

 telegraphy f. Since that publication another more compact 

 form of instrument has been designed by him, adapted for 

 measuring a larger range of oscillation constant. 



The principle on which the above-mentioned appliance 

 works is as follows : — If there be any circuit, open or closed, 

 in which electrical oscillations are taking place, we can set 

 up oscillations of identical frequency in another inductively 

 coupled and adjustable circuit, provided that the mutual 

 inductance of the two circuits is very small. 



If this last circuit can have its inductance (L) and capacity 

 (C) varied, and if the values of these quantities are known, 

 the maximum resonance will be excited when 



p = 27rn = l/ \/CL, 



where n is the frequency in the primary or tested circuit. 

 This simple relation holds good, however, only when the 

 mutual inductance of the two circuits is sufficiently small. 



For the sake of brevity the author calls the quantity 

 V CL the oscillation constant of the circuit. Let us assume 

 then that we have a circuit consisting of an inductance-coil 

 and a condenser in series with each other, the circuit being 

 closed on itself. Let part of this circuit consist of a straight 

 wire or rod which can be placed near to any circuit in whicli 

 oscillations are taking place. Then these last induce others 

 in the first-named circuit. Suppose that means exist for 

 varying together both the inductance (L) and the capacity (C) 

 of this closed circuit ; we can bring it into a condition in 

 which its oscillation constant, viz. VOL, has such a relation 

 to the frequency of the oscillations in the primary circuit 

 that ?i = l/27r V^L, and then the maximum potential dif- 

 ference will exist between the plates of the condenser forming 

 the closed circuit. This critical condition may be ascertained 



* Communicated "by the Physical Society: read March 24, 1905. 



t See .J. A. Fleming, " On an Instrument for the Measurement of the 

 Length of Long- Electric Waves, and also Small Inductances and Capa- 

 cities/' 1'roc. Roy. Soc. Lond. vol. lxxiv. ]>. 488 (1905), 



