. The thermionic vacuum tube and its applications . d condenser C3 serve to main-tain the grid of the amplifier tube at a d-c. potential equal to thatof the negative end of the filament. If necessary, a grid batterycan be inserted to keep the grid of the amplifier negative. Thiscircuit shows two batteries, £&, for supplying the plate second battel y is inserted when it is necessaiy to operatethe amplifier at a higher plate voltage than the detector. The transmitting system shown in Fig. 216 has the advantageof high efficiency, but, on the other hand, the oscillation coil ofthe osc
. The thermionic vacuum tube and its applications . d condenser C3 serve to main-tain the grid of the amplifier tube at a d-c. potential equal to thatof the negative end of the filament. If necessary, a grid batterycan be inserted to keep the grid of the amplifier negative. Thiscircuit shows two batteries, £&, for supplying the plate second battel y is inserted when it is necessaiy to operatethe amplifier at a higher plate voltage than the detector. The transmitting system shown in Fig. 216 has the advantageof high efficiency, but, on the other hand, the oscillation coil ofthe oscillator forms part of the antenna, and therefore the fre- DETECTION OF CURRENTS WITH THE VACUUM TUBE 363 quency of the wave radiated changes with slight changes in theinductance of this coil. A system which is not subject to this disadvantage is shownschematically in Fig. 218. The principle of this system consistsin modulating a low-power radio frequency current and thenamplifying this modulated current before it enters the antenna. Amplifier. Fig. 217. The oscillator 0 may be a vacuum tube oscillator giving the radiocurrent at the desiied frequency. The voltage obtained from thisgenerator and the speech voltage obtained from the transmitterare both impressed on the modulator M. This part of the systemdoes not have to handle large amounts of power. The oscillatortube can be a small t^qje of tube operating on a plate voltage of (0= M A, ^2 To Antenna e= 0 Fig. : IS. 100 to 200 volts or even less, and the modulator can be a tube ofabout the same capacity. The speech voltage obtained from thetelephone transmitter can then be impressed directly on the inputof this modulator without amphfication. The modulated outputis amplified by an amplifier A\, and can, if necessary, be furtheramplified by means of a power tube, or a set of power tubes oper- 364 THERMIONIC VACUUM TUBE ated in parallel. The output from these power tubes Is Impressedon the antenna. It will be evident that in such
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