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 



oHq o , o ' 



PAPER ) 



f 



LENGm AaORWNtrlO CARftClTY 



STBANOtO COPPER WIRE 

 FIG. 2 



=7= 



TmFOIL 



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 



313 



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. 



