Popular Science Monthly
��by the high price of the most expensive types. It is good policy, however, for the student to invest as much as he can spare in good telephones, even if a saving must be effected by cutting down the size of the transmitter.
The Blocking Condenser
Another essential part of the receiving apparatus is a blocking condenser, which is used to prevent the tuning coil from
���Fig. 5. One of the holders for the con- denser made of hard rubber, fiber or wood
short-circuiting the detector or tele- phones. Such a condenser as shown in Figs. 4, 5, and 6 may easily be made. A "fixed condenser" may be purchased from any wireless supply store, but it is a good plan for the experimenter to make one. By doing so not only is the actual construction of the instrument learned, but the weak points which might cause trouble later are located.
A pattern for the tinfoil sheets is cut as shown in Fig. 4, 2 in. square but each having a lug ^4 in. square at the corner. Thirty of these will be needed for the condenser. It is also necessary to cut out about thirty-five sheets of thin paraffin paper 2},4, in. square, as shown by the dash lines in Fig. 4. The con- denser is begun by placing a sheet of paraffin paper upon a fiat surface, and putting on top of it one tinfoil sheet with the lug at the lower left corner, as shown by Y in Fig. 4. On top of this foil is placed a sheet of paraffin paper, and upon it a second sheet of foil; this time the lug is turned to project at the upper right corner, X (dotted lines) in Fig. 4. Then a sheet of paper is added, and upon it a third piece of foil with its lug in the Y (lower left) position. Another sheet of paper is put in place, and then a fourth piece of foil with its
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lug in the X position. Thus paper and foil are alternately added, and the position of the lug changed each time. The result is a pile of thirty sheets of tinfoil separated by thin parafiin paper, fifteen lugs projecting to the left and the fifteen alternate lugs projecting to the right. Care must be taken that none of the alternating sheets of foil touch each other, since this would short-cir- cuit the condenser.
A holder for the paper-and-foil con- denser is made by cutting out two pieces of J/g in. or 3/16 in. hard rubber or fiber or hardwood about 2^ in. by 4 in., and drilling four holes in each as shown in Fig. 5. An 8-32 machine screw is passed through each of these holes, washers being placed between the clamping pieces in such number that the condenser is firmly gripped. The upper right and lower left screws X' and Y' clamp the groups of tinfoil lugs X and F, as shown in Fig. 6, and the binding posts X" and Y" mounted upon their upper ends serve to make electrical connection. The other screws 16 and 16' are merely for mechanical strength.
When the condenser is finished, paraf- fin may be melted and poured in to fill the entire space between the two clamp- ing plates. If the construction has been careful and if the condenser is in good condition, when a dry cell and telephone are connected in series with the binding posts X" and Y" only a very faint click will be heard as the circuit is made and broken. If the condenser is short- circuited (and therefore useless until repaired) the telephone will click as loudly with it in series as when connected directly across the dry cell.
Additional Apparatus In the next article there will be described the buzzer-testing arrange-
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����Fig. 6. The holders with foil and pai>er be- tween them and binding posts on on * surface
ment which is used to adjust the crystal- detector to its sensitive receiving con-
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