Popular Science Monthly 



I. Plates 2 and 4 have no tinfoil coatings. 

 Press firmly together, and bind with 

 friction tape. Connection with the lugs is 

 made by the use of spring clips, which may 

 be bent out of copper ribbon. The con- 

 denser is sealed into a wooden case with 

 paraffin so that it is entirely surrounded by 

 the wax. The bottom of the container 

 should first be covered with melted paraffin. 

 When this cools, put in the con- 

 \ I / denser and fill up to the brim. 



The instruments are arranged compactly and 

 the hookups shown are used to tune the circuit 



Brush discharges (the fine violet spray 

 noticed around the edges of poorly built 

 condensers) are thus eliminated, and this 

 source of energy loss is done away with. 



The helix has twelve turns of ^-in. cop- 

 per ribbon. Necessary dimensions are given 

 in Fig. 2. Great care must be exercised in 

 making the slits . for the ribbon. Clamp 

 pieces A and B side by side and cut them 

 simultaneously with a fine scroll saw. 



In wiring up the set use heavily insulated 

 rubber-covered copper wire not smaller 

 than No. 12. While copper or brass 

 ribbon is an efficient conductor of high 

 tension, high-frequency currents, it will be 

 found rather clumsy to handle. Arrange 

 the instruments compactly, and use the 

 hookups outlined in Fig. 3. Short leads 

 are desirable, but do not jam the instru- 

 ments too close together. 



To true the set to 200 meters, place clips 

 1 and 2 at the points indicated in Fig. 3. 

 The closed circuit is then tuned to 200 

 meters. To bring the open circuit into 

 resonance with it, place clip 3 as indicated 

 and vary the position of clip 4 until a 

 receiving station receives the emitted 

 signals at maximum intensity. The wave- 

 length of the set when tuned in this 

 manner may be 2 or 3 meters out of the 

 way, but this is near enough to the legal 

 requirement for all practical purposes. It 

 must be understood that these directions 

 apply only to this particular set where the 



789 



closed circuit has been calibrated in 

 advance to 200 meters. If a condenser of 

 different capacity or a helix of different 

 dimensions or design than that described is 

 used, these turning instructions become 

 useless. 



Many experimenters experience con- 

 siderable difficulty in adjusting the vibrator 

 on the spark coil. Stiffening the vibrator- 

 spring with a wad of paper placed just 

 behind, but not attached to the soft iron 

 button, is effective in producing a mod- 

 erately high, musical tone. Sometimes the 

 rapidity of make-and-break can be in- 

 creased by demagnetizing the soft iron 

 button. This is heated red hot and 

 plunged into water, the process being 

 repeated several times. Sparking at the 

 contact points, which causes a ragged note, 

 can be minimized by shunting an additional 

 condenser having about 300 sq. in. of tin- 

 foil, across the vibrator. Lin^n paper in- 

 stead of glass is used for the dielectric. Do 

 not try to produce a 500-cycle pitch with 

 the vibrator; you will only wear out the 

 platinum contacts, and your range will fall 

 off considerably. A clear-cut, solid note 

 resembling that of Cape Cod will give the 

 best results. 



Rotary-gaps cannot be used with spark 

 coils. Good quenched gaps are efficient, 

 but they do not give the spark a higher 

 pitch than the ordinary straight gap, and 

 their construction is much too difficult for 

 the average experimenter to undertake 

 with his limited workshop equipment. If 

 not designed with exactness and built with 

 great mechanical precision, very serious 

 energy losses result. At the present time 

 there is no moderately-priced quenched gap 

 suitable for small spark coils on the 

 market. 



Judging from his own experience and 

 from general observation the author is con- 

 vinced that owners of i-in. spark-coil trans- 

 mitters who apply these principles to their 

 equipments will save themselves much 

 time and expense and may feel confident 

 that their senders produce a minimum of 

 interference. — Sumner B. Young. 



Automatic Cut-Outs for Use in 

 Charging Storage Cells 



THOSE who charge storage batteries 

 realize the need of some kind of 

 protection for the batteries during the 

 charge in case the voltage of the charging 

 dynamo should fall below that of the 



