146 



total reactance. The total reactance is 

 found by subtracting the capacity por- 

 tion from the inductance portion, each 

 computed as above. When the con- 

 denser and the inductance are chosen so 

 that they neutralize each other for the 

 operating frequency, the impedance 

 reaches its lowest possible value and 

 equals the simple resistance. This con- 

 dition of balanced reactances, therefore, 

 gives the largest possible current for any 

 applied voltage of the given frequency. 

 The circuit in this condition is in reso- 

 nance, and the frequency for which the 

 capacity and inductance neutralize is 

 the resonant frequency. 



The antenna circuit of Fig. i is in 

 many ways equivalent to the closed cir- 

 cuit of Fig. 6. The aerial itself pos- 



Popular Science Monthly 



sesses capacity, inductance and resist- 

 ance, and the coil B adds to the system 

 inductance and resistance. If the total 

 inductance of the circuit is adjusted by 

 varying coil B so that it exactly neutral- 

 izes the capacity of the antenna for the 

 frequency of the alternator E, the an- 

 tenna will be resonant or tuned to this 

 frequency and the greatest aerial cur- 

 rent will flow. If the inductance is 

 changed, or if the frequency of E is al- 

 tered, the reactance Vv'ill at once com- 

 mence to grow large and by increasing 

 the impedance will cut down the antenna 

 current and the radiated waves. 



In the next article further useful ap- 

 plications of resonance will be described, 

 and additional simple computations ex- 

 plained. 



Recent Radio Inventions 



By A. F. Jackson 



A patent issued during 191 5 to C. D. 

 Ainsworth and bearing number 

 1,145,735 shows an interesting ar- 

 rangement of three-electrode vacuum- 

 tube detector. Fig. i indicates the con- 

 struction of the device and the circuit 

 connections. Referring to this drawing, 

 within an evacuated glass bulb / is sealed 

 a support 8 which carries a tubular 

 anode 2 and two electrodes ./ and 6, also 

 in the form of tubes and concentric with 

 the central conductor. The two outer 

 cylinders are made of woven wire, 4 

 (which may correspond to the grid of 

 an audion) being of somewhat finer 

 mesh than 6. The tube is operated cold, 

 i. e., without a filament heated by auxil- 

 iary current, and secures its conductivity 

 through the radio-active material, such 

 as uranium, which is placed near the 

 electrodes at p. The usual circuits, com- 

 bining antenna and ground with induc- 

 tively coupled secondary coil 10 and 

 tuning condenser //, are used. The 

 central electrode, however, corresponds 

 approximately to the plate in the usual 

 audion arrangement, and is connected to 

 the positive terminal of the battery 5 

 through the telephone 12. No series con- 

 denser in the circuit of electrode 4 is 

 shown. 



The patentee explains the operation of 

 the detector by saying that the rarefied 



gas within the tube is made conductive 

 by the radiation from p, which may be 

 a compound of uranium, thorium, radi- 

 um or actinium, and that consequently 

 a steady small current tends to pass from 

 2 to 6 and to 4. The voltage of /j is 

 adjusted just below that which will 

 "break down the electrical resistance of 



^--Rac//o acf/Ve moferjo/ 

 Fig. 1. An interesting arrangement 

 of three-electrode vacuum-tube 

 detector 



the ionized gas" when no signals are be- 

 ing received ; but when currents are in- 

 duced in the secondary system from the 

 antenna, a re-distribution of potential 

 takes place and the battery flows, so 

 producing a signal in the telephones. 

 This described operation is therefore 



