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.
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