shown in Figure 5, as it would appear if the automatic recorder were to respond uniformly a t all wave lengths. These modifications were designed for the visible region. The application is so simple, however, that most ranges of wave lengths for which the Beckman Model DU spectrophotometer is used may be scanned automatically with minor changes in the constituent parts* The modified SERA is especially valuable for rapid identification of
samples, but may also be used for fairly precise quantitative work despite simplicity of design and inexpensive construction.
T.,Jr., Smith, D. C., Rev. Sci. Instr. 18, 173-81 (1947). (4) Etzel, H. W., J. Opt. SOC.Amer. 43, (3) Coor,
87-90 (1953). (5) Goldring, L. S., Hawes, R. C., Hare,
G. H., Beckman A. O., Stickney, M. E., ANAL. &EM. 25, 869-78
LITERATURE CITED
(1) Beckman Instruments, Inc., Bull. 3 6 0 4 , “Operating Instructions 92300 Spectral Energy Recording Adapter fo;, Model DU Spectro-
photometer.
(2) Beckman Instruments, Inc., Bull. 380, “Spect;d Energy Recording
(1953).
(6) Kaye, W., Devaney, R. G., J. Opt. SOC.Amer. 42, 567-71 (1952). (7) Lewin, S. Z., Fairbanks, R. H., ANAL.
CHEM.27, 2020 (1955). (8) Ro er, G. L., Lawrence, H. C.,
KO-
&ma, S. P., Warren, C. W., Ibid.,
Attachment.
27,501-6 (1955).
Falling-Drop Timing Circuit with Automatic Reset D. J. Fisher, Analytical Instrumentation Group, Analytical Chemistry Division, O a k Ridge National Laboratory, Union Carbide Nuclear Co., O a k Ridge, Tenn.
circuit was designed as a comT ponent of a falling-drop densimeter. Where Stokes law is valid, the time of HIS
fall of a spherical drop between two fixed points is inversely proportional to its density. The drop is caused to fall through two light beams. This circuit can determine the fall time of a drop of clear water as small as 1 pl. and was used routinely to determine the density of 5-p1. sample drops whose density ranged from about 1 to 2 grams per ml. CIRCUIT AND OPERATIONAL SEQUENCE
The power supply is conventional. For greater safety, an isolation transformer should be added. Type 919 phototubes were chosen because they are more stable than gas types. The upper slit phototube circuit is
2afuse
.
direct current-operated. The decrease in light to the upper 919, as the drop falls through the upper light beam, causes the potential of the upper 2D21 grid to approach that of its cathode, so that the upper 2D21 fires. Because it is direct current-operated, the upper 2D21 will stay fired until the normally closed points of relay 2 momentarily open its plate circuit. The firing of the upper 2D21 actuates relay 1, so that the timing clock starts running. The lower slit phototube circuit is alternating current-operated. If the lower 2D21 is fired by a momentary signal to its grid, it will remain fired only until the next negative half-cycle of line voltage extinguishes it. The decrease in light to the lower 919 tube as the drop falls through the lower light beam causes the lower 2D21 to fire during a positive line half-cycle and this actuates relay 2. The actuation of relay 2 opens the plate
circuit of the upper 2D21, which extinguishes it, de-energizes relay 1, and stops the timing clock. The next negative power line half-cycle extinguishes the lower 2D21, which de-energizes relay 2, so that the upper circuit is also automatically reset to its initial state and the circuit is ready to time the fall of the next drop. The constructional details in notes 1 and 3 are intended t o increase the longterm stability of the timing circuit. By means of the four adiustable resistors and by use of a sensitive vacuum tube voltmeter, the bias (difference in potential between pins 1 and 2 ) of the unfired 2D21 tubes is adjusted to a value of -4 volts. The lowest feasible potential to light bulbs L1 and LZ results in the longest possible bulb life.
I
I neutral
UPPER SLIT switch
L 2.7 K 51 4.7 KO
X *X +
2 K (OM51
351
Ira
* X upper slit liqht source
n
*X
w
25w
351
+K51
n
1
lower s l i t liqht source
25w
LOWER SLIT
Figure 1.
Falling-drop timing circuit
Notes 1. L and Lp are Mazda 88, 6-8 volts, 2.04 amperes, or W-87, 15 cp., 6-8 volts; solder leads to base of bulb and insulate 2. R11 and R11 are Sigma 5FLJ2000 G both terminals from ground 3. Cover phototubes with diffusion screen consisting of one layer of Kleenex pasted on with clear plastic spray 4. Adjust to give 4volt bias to unfired 2D21 5. If difficulty is experienced due t o low line voltage, substitute a voltage doubling circuit for d.c. supply circuit shown
308
0
ANALYTICAL CHEMISTRY
