George K. Estok
University of Son Diego Son Diego, California
Adapting the BC-221 Frequency Meter for Dielectric Constant Measurements
The use of a BC-221 frequency meter1 for dielectric constant measurements by introducing a dielectric cell parallel to the variable capacitor of the meter itself has been described earlier.% Although this represents a simple conversion of the instrument, the tuning capacitor is not ideally suited for this purpose. Also, the many beat signals which are heard from the instrument, none of which are at the fundamental crystal frequency, necessitate an inconvenient operating procedure. Described here is a method of adapting the BC-221 without affecting any of the internal wiring of the instrument, by plugging in an external tuned circuit which will oscillate a t 1000 KC (the crystal frequency). The internal tuned circuit is thus not used. The wiring diagram for the adapter is shown iu Figure 1 (bottom views of plugs and sockets), Figure 2 is a suggested arrangement of components, and Figure 3 is the schematic diagram of t,he modified oscillator circuit.
Figure 1 .
Adopter wiring diagram.
Figure 2.
Adapter component arrangement.
The 6SJ7 oscillator tube is removed from the BC-221, and the adapter plugged in its place. The 6557 is then plugged into the adapter socket. The plug-in-type coil of the adapter is designed to operate in conjunction with a General Radio type 722-K precision capacitor, or equivalent (15&1100 mmf). The coil has an outside diameter of 13/8 in., and is wound with about 26 turns of #24 or #26 enameled copper wire, tapped a t about 8 War ~urplusitem. 1
turns from the base of the coil. The exact number of turns should be determined by "pruning" the coil carefully while testing it (with one unsoldered connection) in the apparatus and obtaining a "zero beat" at a convenient position almost at the high end of the 722-N dial. To prevent vibrational effects on the frequency stability the adapter must be securely mounted, and this is acoomplished by the mounting bracket shown on the coil end of the aluminum adapter chassis. Siice the adapted oscillator operates a t the fundamental frequency, the beat note is very loud. A few minor beats may also be heard in tuning the 722-N, but these will be too faint to be confused with the fuudamental. The BC-221 meter, when in its metal box, is very slow in attaining frequency stability because of tube heat; therefore the chassis should be used outside of the box so that heat may dissipate more readily. Even so, the instrument should he permitted to warm up for 3 M 0 minutes until a sufficiently stable zero beat is ohtained. The apparatus may be battery powered (as originally
478 / Journal of Chemical Educufion
Modifled oscillator circuit
intended), or a power supply as earlier suggested2may be used, or a suitable small power supply may be bought nominally from some radio supply houses (industry surplus). The apparatus is suitable for student or research use. Details of operating this type of hetrodyne-beat apparatus for dielectric constant measurements have been published earlier.=
TAOMPSON, H. B., AND RORERS, M. T.,J. CREM.EDUC., 32,
20 (1955).
Figure 3.
a
ESTOK, G . K., Ant. J. Phys., 23, 542 (1955).
