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 



'/91 



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- 



/I5 



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- 



