Page:Popular Science Monthly Volume 91.djvu/805

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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

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��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

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