Page:Popular Science Monthly Volume 88.djvu/177

 Popular Science Monthly

of the receiving coupling C and the sec- ondary of another oscillation transform- er E. The secondary D may have the tuning condenser Q connected across its terminals leading to the detector R and stopping condenser 6". Across this last named are connected the ordinary telephones T and potentiometer with battery, U. In addition to these usual receiving instruments, a generator of radio frequency current is coupled to the oscillation circuits. This may be, as shown in Fig. 4, an oscillating arc / having the resonant condenser H and inductance G connected serially across it and fed with direct current from O through resistance M and choke coils L. L.

The heterodyne operation of this re-

���Fig. 5. Curve indicating operation of new rectifier heterodyne

ceiver may be explained with reference to Fig. 5, which is a series of curves roughly representing the currents in the several circuits. The upper line, Ni, indicates the incoming- wave cur- rents as they would be set up in the antenna and secondary circuits if sig- nals were arriving but the local oscilla- tor were not in operation. The second curve N2 shows the current of slightly different frequency which is generated by the local oscillator itself, as it would be induced in the receiving circuits if no signals were being received. The third curve, N^, represents the beat- current which is produced in the cir- cuits when signals are being received and the local generator is running; this current is seen to change from zero to

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maximum strength regularly, accord- ing to whether the two interacting cur- rents aid or oppose each other. This varying radio-frequency current has a beat frequency equal to the difference of the two radio frequencies, just as in the simple magnetic heterodyne, and, when rectified by the detector R, produces in the telephone circuit a pul- sating direct current corresponding to the heavy curve on axis A'^^. These pulses of course act on the telephone diaphragm in the w^ell known manner and produce a musical signal tone of the beat-frequency.

This recent type of heterodyne is the forerunner of many receivers used to- day for continuous wave signals. In some of these the local oscillator is a suitably arranged audion bulb and the detector a second audion. Occasion- ally amplifiers are added, and a very sensitive receiving system thereby ob- tained. In some instances the same audion bulb is used as a local gener- ator, and, simultaneously, as the de- tector and amplifier. The basic method of operation can be traced back, how- ever, to the heterodyne principles ex- plained in the above three patents and outlined herein.

A Multiple Point Switch

THE drawing shows a positive con- tact, smooth running multi-point switch, having i/^>-inch diameter switch points on %-inch centers, with the width of contact arm 'Vs-inch. It may be seen from the drawing that all movable con- tacts are of the self-cleaning knife edge type. An attractive and substantial in- strument is the result.

This switch may be used on the high voltage audion battery circuit by leaving each alternate contact point dead, and making connection through the central contact ring. This protects the battery against short circuits.

���This switch may be used on a high voltage audion battery circuit

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