488 Prof. J. A. Fleming. On an Instrument for the [Jan. 26, 



" On an Instrument for the Measurement of the Length of Long 

 Electric Waves, and also Small Inductances and Capacities." 

 By J. A. FLEMING, M.A., D.Sc., F.E.S., Professor of Electrical 

 Engineering in University College, London. Eeceived January 

 26, Read February 9, 1905. 



The measurement of the length of the waves used in connection 

 with Hertzian wave telegraphy is an important practical matter. 

 Since the wave-length of the radiated wave is determined hy the 

 frequency of the electric oscillations in the radiator, the determination 

 of this frequency is all that is required. The principle of resonance is 

 generally called into assistance to effect this measurement. It may he 

 done by the employment of either an open or a closed resonant circuit. 



Instruments for this purpose have been devised in which some circuit 

 having capacity (C) as well as inductance (L) in series with each other, 

 has one or both these factors altered until the electrical time-period of the 

 circuit agrees with that of the wave making circuit. Since this agree- 

 ment depends upon the equality of the product JGL for the two 

 circuits, the author proposes to call this quantity ^/CL the oscillation 

 constant of the circuit and the number of oscillations in 2?r seconds or 

 2-Trn, where n is the frequency, the oscillation number. Then it is a 

 property of simple oscillatory circuits that the product of the oscillation 

 number and oscillation constant is unity. Some taeans has, therefore, 

 to be employed to indicate when the adjustment of the two factors of 

 the adjustable circuit has brought its oscillation constant into agreement 

 with that of the transmitter circuit. In a wave-meter devised by 

 J. Donitz,* which is of the closed circuit form, a condenser of variable 

 capacity has its terminals short-circuited by an inductance coil, and 

 this coil is acted upon inductively by some part of the transmitter 

 circuit so that oscillations are set up in it. A variation of the capacity 

 is made until the root-mean-square value of the current in its circuit is 

 a maximum. This is done by the employment of a sensitive form of 

 hot wire ammeter. 



There are, however, some objections to this form of wave-meter, and 

 especially to the use of a hot wire ammeter. The root-mean-square 

 value of the oscillation depends not only upon the maximum value and 

 logarithmic decrement of the oscillations, but upon the number of 

 groups of oscillations per second. Hence, if the discharger of the 

 transmitter is an ordinary spark discharger, the variation of the 

 oscillations due to variation in the break speed or spark-ball surfaces 

 and, therefore of the root-mean-square value of the current set up in 



* See J. Donitz, " On Wave Meters and their Uses," ' Elecktroteohnische Zeit- 

 schrift,' vol. 24, p. 920, 1903, Nov. 5; also 'The Electrician,' vol. 52, p. 407, 

 Jan. 1, 1904. 



