310 



Popular Science Monthly 



they will establish a circuit between the 

 line of contacts A and B. The arm contact 

 which presses on C is connected with the 

 handle swivel and in turn with one of 

 the binding posts. 



It can be seen that when the handle is 

 in the neutral position, no connection is 

 made with the motor. Moving the handle 

 to the right or left causes the motor to ro- 

 tate either one way or the other. This is 

 accomplished through a reversal of the 

 field connections. After a direction of ro- 

 tation is established, the speed is varied by 

 progressing the switch handle from notch 

 to notch on contacts shown at A in the 

 diagram. — K. M. Coggeshall. 



An Ingenious Wiring System for 

 Two Inductive Transformers 



THE accompanying diagram shows a 

 very good wiring system for the 

 experimental set. Five different connec- 

 tions are possible for the two inductive 



Tabulation switch positions 

 circuits shown in the diagram 



for the various 

 of the wiring 



couplers by using five single-pole, double- 

 throw, knife-switches. The inductive 

 couplers are shown in Fig. I and 2, A, the 

 Fessenden interference preventer, having 

 the two primaries connected in multiple and 

 the secondaries in series; B, the selective 

 tuning with the secondary of the first in- 

 ductive coupler connected with the primary 

 of the second; and C, a long wavelength 

 "hook-up" with both the primaries and the 

 secondaries in series. A variable con- 

 denser with short circuiting switch D may 

 be placed in series with the second primary, 

 and another with open circuiting switch E 

 may be added to time the secondary. The 



connecting wires should be of No. 18 gage 

 lamp-cord as short as possible. 



A throw of the switches will tell which of 

 the two transformers is the better. It is 

 a good plan to have two detectors as shown 



CI 



GROUND 



TRANiF 

 FIG Z 



Very satisfactory wiring diagram of two in- 

 ductive couplers for an experimental set 



connected with the switch F for the sake 

 of comparing the different minerals, and 

 also in case one gets "knocked out" in the 

 middle of a message. — Harvey N. Bliss. 



A Variable Condenser for a 

 Radio Receiving Set 



THE average amateur, in constructing 

 his own wireless receiving apparatus, 

 encounters his greatest difficulty in mak- 

 ing a good variable condenser. A simple 

 tubular condenser is shown in the illus- 

 tration. It is easily constructed. The base 

 is preferably of oak, 14 in. long, 3 in. 

 wide and Y2 in. thick. The ends which 

 hold the large cylinder are each 2^ in. 

 square and Y2 in. thick. The brass pipes 

 are 6 in. long and 1 Y2 m - m diameter for the 



ij BRASS PIPt 6*L0Nb 



f RRASS PIPE fc'L0N6 



A" tubular condenser, if properly constructed, 

 will serve just as well as a plate condenser 



large one and 1 in. in diameter for the small 

 one. A wire soldered to the stationary 

 tube is connected with one binding post and 

 a flexible cord which is soldered to the mov- 

 able tube is connected with the other bind- 

 ing post. — Thomas Lee Hodges. 



