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Page:Popular Science Monthly Volume 89.djvu/971

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Popular Science Monthly


��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.

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