638 



Popular Science Monthly 



may be glued in place and should be 

 double, one inside and one outside of 

 each hole. The base shown at C, Fig. 5, 

 is made of J^-in. or i-in. quartered oak 

 stained and waxed, and drilled as shown 

 to correspond to the motor base and gap 

 casing. ' For assembling there will be 

 needed four round head brass screws to 

 fit the thumb nuts of Fig. 2, 2 flat head 

 wood screws about i>^ in. long, one 3/16- 

 in. iron machine screw about i^ in. long 

 having fiat head and a nut and washer, 

 and a dozen J^ in. or ^ in. round head 

 brass wood screws. 



In Fig. 6 are seen three views of the 

 complete gap. The wood block should 

 be mounted on the base by means of the 

 i^-in. wood screws and the 3/16-in. bolt, 

 the flat head being on the inside of the 

 block. Then the motor should be 

 mounted in place on the base, with its 

 shaft extending into the casing. The 

 electrode bushings should be put in 

 place on the sides as shown, using the 4 

 screws whose heads may need to be filed 

 flat to about 1/32 in. thickness, and the 

 thumb nuts and hexagon, or round nuts, 

 as binding posts. 



The electrodes may then be placed in 

 the bushings and the wooden handles put 

 on. Last of all, the rotor should be 

 firmly fastened on the motor shaft with 

 its set screw and the fiber front should be 

 screwed on with the J^-in. round head 

 brass screws. {B, Fig. 6.) Then the 

 gap is complete. 



To insure the desk or table top 

 against any slight vibration arising from 

 the motor, blocks i in. square of sponge 

 rubber should be placed under each 

 corner of the base. 



Before starting the gap be sure to 

 have the electrodes fastened in the 

 proper places with the set screws in the 

 bushings. The gap described here is 

 sufficiently large for a i -kilowatt outfit 

 using 10,000 to 20,000 volts. If the 

 constructor has access to a milling 

 machine the casting can be much 

 smaller; but the method shown in Fig. i 

 will give satisfaction. 



A Simple Method of Measuring 

 Capacity of an Antenna 



REFERRING to the diagram of con- 

 nections, it will be seen that £ is a 

 buzzer; F, one or two dry cells and / a 

 cr>'stal detector. / represents the tele- 



phones; G is a variable condenser of 

 known values (i. e., calibrated) and H is 

 the condenser which is to be calibrated 

 and whose capacity is unknown. A, B, 

 C, D are four equal inductances as 

 described below. 



This method comes within the scope of 

 material and instruments which the 

 average experimenter has on hand, and 

 is also very accurate. 



The detector, buzzer and telephones 

 are of any of the familiar types. The 





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Diagram of connections for the apparatus 

 to measure capacity of antenna wires 



inductance coils should be made by 

 winding four separate and similar coils. 



Each one is to be made as follows, or 

 as nearly so as may be convenient with 

 the wire, tubes, etc., on hand. 



Upon cardboard tubes 2 in. in diam. 

 and i^ in. wide, wind twenty-five turns 

 of No. 22 single copper-covered wire. 

 Coils B and D are to be connected 

 in the opposite direction to the wind- 

 ing on coils A and C, so that the two 

 circuits will be opposed. 



After connecting in the condenser G 

 to be measured, turn the handle of 

 variable condenser G until no signals are 

 heard in the telephones.. Then the 

 capacity of H will be equal to the 

 capacity of G, which of course* must be 

 ascertained from the calibration curve 

 sheet. The operation should be per- 

 formed very slowly as the "no signal" 

 point is very sharp and it is very easy to 

 pass over it. 



In ordinary operation the coils A and 

 C should be kept at least 3 or 4 in. 

 from coils B and D, so that the coupling 

 will not be too close. It will be noticed 

 that on all other points of condenser G 

 a continuous signal will be heard from 

 the buzzer, 



A calibrated condenser covering a 

 range from .0002 to .0025 or even .003 

 microfarads is a good size to have. 



