Popular Science Monthly
��adjustments, or because they do not retune primaty or secondary circuits after chang- ing coupling values. Practice with the apparatus of Fig. 7 should make the desirability of correct tuning evident to every experimenter.
In the next article of this series there will be discussed the secondar\' variable con- denser. When properly used, this instru- ment is of great value in increasing sharpness of tuning.
��An Easily Made Adjustable Telephone Condenser
A CONDENSER which has the advan- tage of easy variability and inter- change of units may be made as follows: Secure a number of burned-out cartridge fuses and take the caps off very carefully, being careful not to bruise them. Clean out the insides thoroughly, and open the holes in the caps so that you may solder the internal leads to them.
Cut some tinfoil strips 1^4 in. wide and of length to be determined by the capacity of condenser-units you desire. Prepare also some strips of paraffin-impregnated paper about }/2 in- wider than the foil and about I in. longer. Lay down the paper on a smooth surface, and on it place a strip of foil (see Fig. 2) with a piece of stranded copper wire attached to the end for connec- tion. Put a second strip of paper on top of the foil, and then another strip of foil. Fasten a wire terminal to this second foil piece, taking it out in the other direction
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��Above: The cartridge holder brass strip. Be- low : Strips of tinfoil on the paper before rolling
as shown. Now roll up the whole four strips as tight as possible and tie the roll with thread. Soak the rolled unit in merited paraffin until no air bubbles can be seen.
Next insert each rolled condenser into a fuse-tube, with one of the connecting wires
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projecting out of each end. Run these wires through the holes in the caps, and solder. This completes the condenser- units. The capacity-, or the number of square inches of tinfoil, should be marked. A holder for the cartridges should be
���The cartridges filled with the rolls of tin- foil and mounted on the holder or base
made by bending and cutting two strips of I /32-in. spring brass or copper, about No. 20 gage, each of which is 5^ in. long and 1 34 in. wide, as shown in Fig. i and 3. Additional dimensions and the method of mounting are shown in the drawings.
��How to Keep the Telephones from Humming
AMATEURS who have akernating cur- rent in their homes and a droplight near the instruments are usually bothered by a 60-cycle buzz in the telephones. This is very unpleasant when receiving weak signals from a distant station. It will be noticed that by putting the hand near the lamp-cord the sound of the buzz is increas- ed, but when the hand is placed around the cord the sound ceases entirely.
Evidently the body acts as a conductor between the line and the ground. The fol- lowing remedy has proved ver>' satisfactory.
Wind one or two layers of No. 18 or No. 24 cotton-covered wire around the lamp-cord for about 4 or 5 in., keeping them as close and compact as possible. The other end of the wire is then connected to the instrument ground-binding post or some other ground connection as near to the instrument as possible, A telephone or other condenser of i or 2 M. F. connected in series with the wire and the ground also gives good results, and if desired, two tele- phone condensers may be connected in multiple, for the more capacity the better. Do not let the cotton-covered wire touch the socket, as there may be something wrong inside. This method is not good for preventing a buzz caused by arc-lights or other outside circuit. — D. Browne.
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