-I+ c4
11'
,
Figure 2. Circuit diagram for power supply. A = +6.8 v., B = + 1 .O v., C = reference ground, D = - 1 .O v., E = chassis ground
Q1
Qa
Qa
PI
Ne 5 1 pilot lamp F1 1-amp. 3 A G fuse RI 1 5 0 K, '/,-wait resistor 5 K, wire wound Dotentiometer R2 Rs 150-ohm, 1 -watt resistor TI 12.6-v. C.T., 2-amp., filament transformer, Triad No. F44X X I , X Z Silicon rectifiers, Sorkes M 5 0 0 Zener diode, 6.8-v., 1 0-watt, Hoffman No. 1 N 1 8 0 5 X* 1, 10-hry., 50-ma., 500-ohm, filter choke, Triad No. C3X C1, Cp 1 000-pf., 15-v., capacitor, Mallory WP 2 0 0 2000-pf., 15-v., capacitor, Mallory, WP 0 3 9 CJ C, 500-ppf., disk, ceramic capacitor Swl S.P.S.T. toggle switch
may actually introduce a serious error in calculated values of E , especially if in switching from one capacitor to another the deviations work in the opposite direction. The best capacitor for Cs was found by trial and error, with linearity being ascertained by means of the instrument itself. T o avoid the nonlinear portions that usually lie at the ends of variable linear capacitors, a-e used the center section only of a 20-ppf. capacitor in order to get a 10-ppf. linear section. To check for linearity we determined the difference in capacitance of C x a t positions A and B for air, using slightly different settings of C9 for each determination. Positions -4 and B are fixed stops set to permit a reproducible rotation of C x through an angle of 180". I n this manner different sections of the vernier dial (and therefore different 2162
Figure 3. Oscillator circuit (left), detector and voltage amplifier circuit (right), and resonance tuning circuit (bottom); connect to power supply at A, B, C, and D
ANALYTICAL CHEMISTRY
R, R6
R6, RE R7
2 N 2 7 4 transitor 2 N 1 0 9 fransisior 2 N 6 4 7 transistor 1 0 0 K, '/Z-wati resistor 1 K, '/*-watt resistor 47 K, '/*-watt resistor 2 0 0 K potentiometer (balance)
Ro
MI XI Xg,
X6
Lz,
13
I, Ca
c6
1 0 M potentiometer (sensitivity) 0-1 00 microammeter 5.0 Mc. crystal CK705, germanium diodes 1 mhry., R.F. choke 1 6 turns, 1 '/,-inch dia., alr wound coil, Air Dux No. 101 OT, 1 Ot./inch, 1"s inches long 10-1 00 ppf.. variable ceramic capacitor, Centralab No. 8 2 3 BN
5 0 ppf., silver mica capacitor 2 4 0 ppf., tubular ceramic Capacitor Variable celi capacitor, 1 0 ppf. (modified b y removing all but 2 rotor and 2 stator Cx plates to give a capacitance o f about 2.4 ppf. between plates-open and platesclosed positions in air), Johnson No. 160-1 0 4 Variable precision capacitor, 19.6 ppf., Johnson No. 160-1 10 Cg Clo-zl Solid-siaie silver-porcelain capacitors, Vitraman VY axial radial series wiih =t1 % tolerance, values in ppf. Variable capacitor, 1 0 ppf., Johnson No. 1 8 9 - 4 Cp 2 0 0 0 ppf., tubular ceramic capacitor Cps 1 0 0 ppf., silver mica capacitor Cz( 2 pf., minoture electrolytic capacitor, 15 v. Czs S.P. 1 2 position, nonshorting, ceramic rotary swikh Swz C7
sections of the center section of CS) were used for tuning. I n our instrument there was less than +0.5% variation from one end of the center section to the other. It is possible to use the manufacturer's rated value of CS in determining the capacitance per division of the vernier dial, but by experience we found that we could ascertain a slightly better value from measurements made with purified liquids of known dielectric constants. I n our instrument, C8 showed a capaci-
tance change of 0.196 ppf. per scale division, instead of 0.200; this constant will be referred to as K . Since the transistors Qz and Q3 function as a temperature-compensated voltage amplifier, they should be mounted in close proximity to each other. I n order t o minimize stray and variable capacitance in connection with the use of the cell, capacitor C X was rigidly attached to the chassis with attention paid to shielding and short heavy leads. The chassis should be
well grounded ( E in Figure 2). If inductance L4 is not mounted externally, close proximity to surrounding metal should be avoided. I n order t o thermostat the liquid when its dielectric constant was being meamred, a simple unshielded water-jacketed cell (8-ml. capacity) was provided for simple insertion and removal. When construction is complete certain internal tuning adjustments should be made. For this purpose follow the first two steps in the instruction summary and then: (1) adjust Rz until the potential between B and C is equal to the potential between C and D; this potential should be about 1 volt and the measurement should be made with a VTVM; (2) adjust Rp until a mid-scale reading is obtained on microammeter MI: (3) adjust Cg carefully for nearly maximum defiection on MI using slightly less c;tpacitance than necessary for the peak, and readjust Ra if necessary to keep the needle on scale; (4) unplug -Y4and adjust R, to zero reading on . l l l ; and (ti) replace Xd and readjust Rs for approximately 90 pa. at resonance. Removc: X4 from socket and note that X1should fall to zero. Any reading indicates self-oscillation and Rgshould be reduced slightly. Fine tuning is achieved by finding the smallest value of Ca wl-ich will keep the microammeter indicator needle a t a fixed stable position near the right end of the scale. This value is determined by rotating the knob of Cs from a value which is too high to one which is too low; just at the threshold of resonance the needle will make a small movement to the left, but this movement should not be confused n-ith the large abrupt deflection to the left ahich corresponds t o the establishmeni, of resonance. When near resonance; the needle will oscillate from side to side with a period which depends on the care with which resonance is establishe(j. A t resonance the period of oscillation is about 10 seconds; at about 1.3 ppf. below the proper resonance point the period is about 4 seconds and the amplitude is much smaller. If the capacitance is more than about 1.2 ihpf. too low, the needle simply rests a t the left end of the scale. USE OF INSTFUMENT
This instrument utilizes the capacitysubstitution principli: described by Bender ( I ) c r a standard text (2). For a given dieiectric in C X the resonance capacitance (C) of the :ircuit is given by
c
= CXB
+
Z
