Popular Science Monthly 



957 



would give a signal ten times as loud as 

 the interference. Under the above 

 assumptions this new receiver would 

 produce a signal thousands of times as 

 loud as the interference. In times of 

 heavy atmospheric disturbance, or when 

 the interference is from powerful nearby 

 transmitters it is probable that favorable 

 adjustment of so delicate an amplifying 

 receiver would be exceedingly difficult 

 to maintain. The device should, how- 

 ever, be useful for many other less 

 trying conditions. 



A New Direct- Current Transmitter 

 for Radio Communication 



THE use of a commutator for produc- 

 ing radio frecjuency alternating cur- 

 rent from direct current, in a wireless 

 telegraph or telephone sending station, 

 is suggested in U. S. Patent No. 1,172,- 

 017, issued to R. A. Fessenden in 1916. 

 The method involves making and break- 

 ing a battery circuit, leading to the 

 antenna and ground system, several 

 hundred thousand times per second even 

 for the generation of long waves. 



It would be very difficult, if not im- 

 possible mechanically, to build a rotating 

 commutator which would operate at 

 such enormous speeds, and the patentee 

 therefore suggests that the commutator 

 itself should remain stationary while the 

 flexible contact brush revolves. If the 

 instrument is built with a diameter of 

 about 8 in. and has segments 1/16 in. 

 wide, there will be room for about 400 

 contacts around the periphery. If the 

 contact brush is driven at a speed of 

 30,000 revolutions per minute (which is 

 not higher than the velocities reached 

 by De Laval turbines and certain centrif- 

 ugal machines) electromagnetic waves 

 of 100,000 alternations per second fre- 

 quency can be generated. This corre- 

 sponds to a wavelength of about 6,000 

 meters. 



One set of circuits shown in the patent 

 is given in the accompanying diagram. 

 The generating commutator is formed of 

 segments 8 and 7 placed side by side 

 and insulated by the separating material 

 9. A brush with flexible tip 5, balanced 

 by the weight 6, is revolved by power 

 transmitted through the belt 5a. The 

 antenna I is connected through tuning 

 condenser 3 and inductance 4 to the 



rotation contact; alternate commutator 

 bars are connected to the opposite 

 terminals of the charging source 13, 14 

 (which may be either two high voltage 

 generators, or batteries, as shown). 

 The middle point of the power supply is 

 grounded for radio frequency currents, 

 from 15 through the condenser 16 to 2. 

 An additional tuning circuit consisting 



The revolving brush produces 

 radio frequency current 



of condenser 12 and coil 11 is shunted 

 across the main power leads, and a 

 variable inductive resistance is placed 

 in circuit at 10. 



Assuming that the upper terminal of 

 14 is of positive polarity, and the lower 

 end of 13 negative, the operation of the 

 transmitter may be outlined briefly by 

 pointing out that each time the contact 

 rests upon a bar or the group numbered 

 8 the antenna system is charged with a 

 positive pulse; when the brush passes to 

 the next contact this charge rushes to 

 earth and the antenna assumes a nega- 

 tive potential. By adjusting the tuning 

 of the antenna circuit as a whole so as 

 to agree with the rate of interruptions of 

 the commutator, strong radio frequency 

 currents can be set up in the aerial and 

 correspondingly intense waves radiated 

 therefrom. 



The diagram merely indicates the 

 basis of the method proposed. Difficul- 

 ties of insulation would suggest the use 

 of two commutators with separate 

 brushes contacting alternately, one for 

 each polarity of charge. Various other 

 modifications of mechanical structure 

 occur in designing a commutator generat- 

 ing machine for regular use. It seems 

 entirely possible that the structural 

 difficulties in the way of building such 

 an alternator would be less than those 

 involved in radio frequency dynamo 

 machines of other types. 



