Excerpt from Methods of Measurement of Properties of Electrical Insulating Materials
Figure 13 shows the complete apparatus as assembled in the radio laboratory of the Bureau of Standards. For' more citact measurements the complete apparatus itself is housed in a small sectio... n of the room and completely surrounded with a fine mesh wire, such as window screening. The purpose of this shielding is to insure the accuracy of measurement by shielding out the in¿uence of outside radio-frequency generators. This View shows the wave meter mounted on a tall stand, 'a rather complicated radio-frequency generator, a rack of I oo-volt' storage batteries, the bottle of distilled water for use with the batteries, the gal vanometer rack showing the scale, lamp and galvanometer, the test Specimen ¿oated ready for measurement, the special skeleton switch, the standard inductor, the standard condenser, and a box of noninductive resistors. In addition there will be observed a series of parallel wires between the generating circuit and the measuring circuit and also between the measuring circuit and the place occupied by the operator. The 'parallel wires between the generating circuit and the measuring circuit are all soldered together at the end nearest the operator (the leftehand end, as shown) and left separate at the other end. The wire which connects all the parallel screening wires is then attached to a gmund connection. The parallel wires between the measuring circuit and the Operator are unattached at either end' but all attached in the middle. This middle connector is also attached to the ground connection. The upper half of the shield between the operator and the measuring circuit is hinged to make it more easy to change the apparatus. It will be noticed that two panels of the over-all wire netting screen have been removed in the photograph. The third panel at the extreme right front of the photograph is hinged to form a door. This door permits the operator to arrange the test specimen in the mercury bath. Method of measurement - Assume that the I3 - turn inductor Lg is in position as shown in Figure 4 and the other parts of the generating circuit are in readiness and generating radio-frequency current. Bring the generating circuit near to the measuring circuit with the shield of parallel Wires between them, as shown in' Figure I 3. Put the -millihenry standard inductor L (fig. 7) in position and insert heavy copper links between wells I and 3 and between 2 and 4. Write the value of the inductance L in space No. I of the data sheet, Table I. Tune the generating circuit to the measuring circuit by varying the setting of the con denser C (fig. Resonance being determined by the maximum de¿ection of the galvanometer G (fig. It is essential that this tuning be done carefully. Change the coupling between Lg and L (fig. 4) until the maximum galvanometer de¿ection at resonance is about 40 cm if a 50-cm scale is used. Leave the generating circuit stand untouched and determine the wave fre quency (wave length) emitted by the generating circuit by means of a frequency meter (wa've meter). Record the wave-meter48 Scientific Papers of the Bureau of Standards.
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