624 



Popular Science Monthly 



Fig. 2. Type of rotary-gap 

 wheel which eliminated a 

 very ragged sounding spark 



each glass sheet, with its two coatings of 

 metal, i4 '"• from tlie next sheet, in a 

 small wooden ruck, and then immersing 

 the whole in a tank of transformer oil. 

 See Fig. i. 



I was then troubled with a "dragging" 

 spark. Judging from some of the sparks 

 I hear every night, others are troubled 

 in the same way, especially those who 



are using 



DIRECTION OF ROTATION high Volt- 



JMiNUM trans- 



7HAR0 fSe trans 



/RUBBER formers. I 

 got rid of 

 this trail- 

 ing and cut 

 out the 

 ragged 

 sounding 

 spark by 

 making 

 my rotary- 

 gap wheel 

 as shown 

 in Fig. 2. 

 This spark 

 break ob- 

 tained by the use of this type of wheel 

 entirely eliminated the pulhng out of 

 the spark, which had been the cause of 

 my ragged tone. 



Every little while, however, the arma- 

 ture of my motor would burn out on 

 account of an electrostatic kick-back. I 

 tried many kick-back preventers, and 

 many different motors, until I finally 

 tried an induction motor. This type 

 of motor has no revolving coil to be 

 burnt out, and consequently this problem 

 was soU'ed. 



I was not yet rid of the troublesome 

 kick-back effects, however, for whenever 

 I sent for any considerable time I would 

 burn out a few scconchiry sections of my 

 transformer. To stop this annoyance, 

 1 made two secondary choke coils, as 

 shown in I'"ig. 3. ICach consisted simply 

 of a single layer of about 50 turns of 

 No. 18 wire wound on a threaded 

 si)in{llc of hard rubber. One coil was 

 comH-ctcd in scries with each secondary 

 lead. 



The aclicjn of the coils chokes back the 

 dangerous high -free] ucncy surges origi- 

 nating in the condenser, yet allows the 

 !ow-fre(]ui-ncy currents from the trans- 

 former to pass through them with ea.sc. 



I have found that a low-pitched tone 

 of about 4H0 sparks per second will carry 

 farther and can be read more easily at 

 a distance than the more common high 

 tone. Other advantages of a low dis- 

 charge rale are that time is given for the 

 condenser to charge and discharge fully, 

 and also that the points on the re\olving 

 wheel do not come opposite the station- 

 ary electrodes so rapidly that a back 

 surge from the oscillation-transformer 

 may jump across the gap, and be wasted 

 instead of continuing in the aerial. 



The most important thing in doing 

 long distance amateur work is tuning. 

 To get good results, the sending set must 

 be in perfect resonance. The condenser 

 must be of correct size for the spark- 

 frequency used; the primary circuit 

 must be tuned to exactly the wave 

 desired, and the secondary- circuit must 

 be put into exact resonance with the 

 primary circuit by the use of a hot-wire 

 ammeter. 



To adjust the whole set to resonance, 

 a procedure such as the following should 

 be used. The transformer should be 

 disconnected from the condenser and 

 oscillation-transformer, and a short 

 straight gap connected across its secon- 

 dary terminals. The aerial should be 

 connected to one side of this gap, and 

 the ground to the other. An arc is then 

 started across the gap and the funda- 

 mental wave length of the aerial is 

 determined by a wave-meter. For the 

 best results it should be about 175 

 meters. 



The smallest condenser that will give 

 a clear spark should then be connected 

 in the circuit with the rotary gap and 

 oscillation-transformer primary. By the 



N°18 COPPER WIRE 



Choke coil consisting of a sinRic layer of 



about 50 turns of wire wound on a spindle 



of hard rubber 



use of the wave-meter, the number i>f 

 turns of primary should be adjusted 

 until the wave is 200 meters. The 

 secondary of the oscillation-transformer 

 should then be connocted with the 



