Page:Popular Science Monthly Volume 88.djvu/174

 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

�� �