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 



149 



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 



